feat: add bounded admin WebSocket backend (Phase 8D.5)

- Require current admin cookie sessions, Origin checks and single-use
  tickets
- Reuse the shared console with session-aware authorization and slot
  allocation
- Add HTTPD-owned I/O, bounded buffering and revocation cleanup
- Prevent LRU eviction of serial clients and stale admin socket closure
- Reject unsupported web-shell mutations before side effects
- Add host regressions, a smoke client and resource accounting

Validated by user sign-off after a 15-minute full-client soak at 230400
baud, with a few broker drops under heavy output. Browser UI remains
for Phase 8D.6; numeric memory reserves remain open.

docs/agent/architecture.md

@@ -98,9 +98,9 @@ TinyUSB callbacks enqueue/copy data and state; the transport task owns broker li
 
 `web_server` runs HTTPS only on port 443 using the device-specific self-signed P-256 certificate from `web_security`. Current routes provide the UI, static assets, status, ticket issuance, and serial WebSocket upgrade.
 
-HTTPS login uses `user_database` and opaque server-side cookie sessions; Basic authentication and its cache are removed in 8D.3. Before administrator bootstrap, the migrated role-`user` account is synchronized from the legacy credential, so that username/password can authenticate through the database; after bootstrap, the legacy blob is independent recovery material and is no longer consulted for authentication or synchronized into role-based accounts. Both `user` and `admin` roles currently receive the same web status/terminal experience; web administration is not implemented.
+HTTPS login uses `user_database` and opaque server-side cookie sessions; Basic authentication and its cache are removed in 8D.3. Before administrator bootstrap, the migrated role-`user` account is synchronized from the legacy credential, so that username/password can authenticate through the database; after bootstrap, the legacy blob is independent recovery material and is no longer consulted for authentication or synchronized into role-based accounts. Both roles retain the same shipped web status/serial UI. 8D.5 adds an admin-only backend without a normal UI entry.
 
-`web_cookie_auth` owns login/session/logout policy: four 120-second digest-only pre-login challenges, explicit same-origin bootstrap, five credential verifications per 60-second global window, and no live-record eviction. Host-only `__Host-` Secure/HttpOnly/SameSite=Strict cookies have absolute lifetimes. Login consumes a challenge, validates bounded JSON and issues a fresh session; logout invalidates only its originating session. Mutations require CSRF and strict canonical HTTPS Origin; serial upgrade requires matching cookie/Origin/ticket. Neither role has web administration yet.
+`web_cookie_auth` owns login/session/logout policy: four 120-second digest-only pre-login challenges, explicit same-origin bootstrap, five credential verifications per 60-second global window, and no live-record eviction. Host-only `__Host-` Secure/HttpOnly/SameSite=Strict cookies have absolute lifetimes. Login consumes a challenge, validates bounded JSON and issues a fresh session; logout invalidates only its originating session. Mutations require CSRF and strict canonical HTTPS Origin; serial/admin upgrades require matching cookie/Origin/ticket, with admin role additionally required by the admin endpoints.
 
 `web_session_store` holds four static records with token/origin digests, copied principal, separate CSRF state, one-hour absolute expiry and non-reused 64-bit session IDs. These are live cookie sessions in 8D.3, with no sliding renewal. A portMUX protects short state copies/mutations; database/RNG/SHA calls occur outside it. Resolution rechecks ID/expiry after database validation; issuance also checks an invalidation epoch. Stop wipes records without resetting IDs/epochs. Only admitted HTTPS starts initialize the store; failed starts and accepted stops disable it before cleanup. Authentication/store-init failure now gates HTTPS startup rather than falling back to Basic. Sensitive views must be wiped by callers; snapshots contain only counts and storage sizes. Focused host checks live in `tests/web_session_store/`.
 
@@ -114,6 +114,10 @@ Web serial initialization is failure-isolated from the base HTTPS service: if th
 
 `web_ui.c` contains authored index/application strings and response policy; it validates `/api/session` before connect/restore, adds explicit Sign out, and cancels stale work on 401/logout/page exit. `web_login_ui` is a standalone public page without protected-asset dependencies. Both authentication documents and app script are no-store. Its restrictive CSP contains a hard-coded hash of the inline loader, so those two must change atomically; preserve same-origin connections, no-referrer behavior, frame denial, and the existing cache policy. `web_assets_data.c` contains checked-in generated arrays for vendored compressed xterm assets and the logo. Normal builds compile these arrays directly; they do not regenerate assets.
 
+### Browser admin backend
+
+8D.5 additionally supplies `web_admin_transport` and `web_admin_tickets`: one optional admin socket, two 30-second digest-only tickets bound to current originating session/principal, the same two shared console slots, no serial broker client. Ticket POST requires cookie/Origin/CSRF/admin; ordinary GET upgrade requires cookie/Origin/admin/ticket and console admission before 101. Six total HTTPS sockets remain, LRU purge is disabled, and two routes bring the handler budget to 16. Optional admin registration/PSRAM allocation failures do not take down M1. A 20 ms ESP timer queues at most one HTTPD poll, with no new task; only HTTPD accesses the 1,552-byte PSRAM-only RX/TX payload or socket IO. Notifiers close the generation-qualified console and flag the socket. HTTPD shuts down the verified current fd directly and owns subsequent read cleanup, avoiding IDF's queued reusable `sock_db *` close race. Detach fences submissions; failed stop retains ownership, and queued state is retired only after successful HTTPD stop. Console dispatcher/prompt and owner input/output/idle checks enforce session and principal currentness. WEB supports deferred self-close only; parsed canonical policy denies unsupported lifecycle/network/account mutations before handler side effects. No normal UI entry, typed settings or lifecycle parity is included. See `docs/phase8d5_implementation.md` for validation limits and exact restrictions.
+
 ### SSH
 
 `ssh_transport` uses wolfSSH on port 22 with two fixed session/handshake slots. Initialization calls `wolfSSH_Init()` in the caller before task creation; after that, one owner task pinned to core 1 exclusively owns runtime contexts/sessions and wolfSSH calls. It enforces bounded handshakes, authentication attempts, receive work, and session buffers.
@@ -157,7 +161,7 @@ admin SSH line editor ----/                              |
 
 The dispatcher is the sole caller of `esp_console_run()`, serializing UART0 and all admin SSH commands. This is required because the console registry is treated as non-reentrant, but it also means a long command or interactive prompt blocks all administration entry routes.
 
-Phase 8D.4 retains this module and exposes `admin_ssh_console_open_owned()`: copied transport-qualified slot/session/generation identity plus a firmware-lifetime immutable owner adapter. The existing two console slots are shared, not multiplied per frontend; active/executing slots cannot be replaced. Owners serialize per-session input, consume output and enforce transport liveness; completion scratch is claimed nonblockingly across owners. The existing control task calls drain/lifecycle adapters outside console locks. SSH implements its adapter and compatible admission entry point in `ssh_transport.c`, using generation-checked published snapshots and existing control APIs, never wolfSSH from the control task. `SELF_CLOSE` is owner-relative; legacy SSH actions remain SSH-specific and unsupported owner actions are rejected. No browser admin transport is yet implemented.
+The 8D.4/8D.5 boundary retains `admin_ssh_console_open_owned()` and adds available-slot admission for runtime SSH/browser owners: copied transport-qualified slot/session/generation identity plus a firmware-lifetime immutable owner adapter. The existing two console slots are shared, not multiplied per frontend; active/executing slots cannot be replaced. Owners serialize per-session input, consume output and enforce transport liveness; completion scratch is claimed nonblockingly across owners. The existing control task calls drain/lifecycle adapters outside console locks. SSH uses generation-checked published snapshots, principal copies and its assigned console index, never wolfSSH from the control task. `SELF_CLOSE` is owner-relative; legacy SSH actions remain SSH-specific and unsupported owner actions are rejected. Dispatcher-side owner `is_current` checks run outside console locks, with full identity recheck after validation. Commands revalidate immediately before the runner; prompts revalidate before publication and after waits (250 ms polling plus check/scheduling latency), rejecting revoked submitted input and stale wakes. SSH preserves close intent through external-close consumption. Consumed output is wiped. These checks do not cancel arbitrary executing handlers or replace owner-side input/output and lifecycle validation.
 
 For SSH, standard output/error is redirected to the invoking session's bounded output ring. `console_input` routes visible or hidden prompts to UART0 or the active SSH session. `exit` and Ctrl+D on an empty admin SSH line use bounded deferred self-disconnect after their acknowledgement drains; role-`user` SSH remains a binary-transparent serial stream. Session tokens include slot and generation so late queued work cannot attach to a reused SSH slot. Only the SSH owner task moves ring output through wolfSSH.