Add air quality history charts

esphome/components/air_quality_web/README.md

@@ -2,8 +2,8 @@
 
 Local external component for **ESPHome 2026.10.2, ESP32 with ESP-IDF**. No remote
 libraries, standard `web_server`, entity enumeration, SSE, OTA, configuration,
-authentication, or history. Only the three explicitly referenced sensor states
-are published. Other device services are not secured by this component. The
+or authentication. Only the three explicitly referenced sensor states and their
+10-hour air-quality history are published. Other device services are not secured by this component. The
 optional mDNS hostname override affects the device-wide mDNS responder.
 
 ## Integration (in the parent configuration)
@@ -61,16 +61,36 @@ uses its native mDNS library; it does not initialize a second responder.
 * Exact `GET /api/air-quality`: JSON with **exactly** `co2`, `temperature`,
   `humidity`, each a finite number or `null`. Example:
   `{"co2":850,"temperature":22.5,"humidity":46}`.
+* Exact `GET /api/air-quality/history`: JSON containing `duration_seconds: 36000`
+  and `samples`, ordered oldest first. Each sample is
+  `[age_seconds, co2, temperature, humidity]`, with nonnegative ages relative to
+  the response snapshot and finite values or `null`. Only samples within the
+  last 10 hours are returned; there are at most 601 samples.
 * Non-GET methods on those paths: 405 with `Allow: GET`.
 * All other paths: 404, including `/events`, `/update`, `/config`, general sensor
   APIs, trailing slashes, encoded aliases, and query-string variants. No body
   parsing or upload handlers. HTTP parser errors can still be rejected by IDF
   before application routing.
 
-The browser polls five seconds after each completed request (4-second timeout),
-never concurrently. Missing measurements show dashes. Network/JSON errors clear
-old measurements and display an offline message. CO₂ is orange **only above**
-1200 ppm and red **only above** 1600 ppm; text accompanies the colors.
+The browser refreshes live readings approximately every five seconds and history
+approximately every minute, using one sequential request loop and a separate
+4-second timeout for each request. Missing live measurements show dashes. A live
+request failure clears the live cards; a history failure retains previously
+loaded history with a stale/unavailable warning, without clearing healthy live
+cards. CO₂ is orange **only above** 1200 ppm and red **only above** 1600 ppm;
+text accompanies the colors.
+
+Three separate responsive charts show CO₂ (ppm), temperature (°C), and relative
+humidity (%) on a fixed −10 h to now axis, with independent automatic Y scales.
+Missing values and timestamp gaps over 90 seconds break the traces. Partial
+history is not stretched across the time axis or extrapolated to the present.
+CO₂ guides at 1200 and 1600 ppm appear when within the chart's value range.
+
+The device records one sample every 60 seconds, independently of browser, VPN,
+or Home Assistant connections. History is kept only in RAM and starts empty
+following reboot; firmware updates therefore also clear it. There are no flash
+writes for history. Delayed sampling is represented by actual timestamps, not
+fabricated catch-up samples.
 
 ## Isolation, security, and resource use
 
@@ -119,7 +139,13 @@ IDF's own timeouts for receiving headers or sending responses.
 A fixed 1-second `PollingComponent` serializes only the three configured sensors
 on the main loop. A mutex protects replacement and copying of the cached JSON;
 HTTP sends happen after releasing it. The HTTP task never reads live sensors or
-the entity registry. Uninitialized, missing, NaN and infinite states become
+the entity registry. The history ring uses 601 compact 16-byte samples (9,616
+bytes), plus small bookkeeping fields. Each history request allocates a bounded
+heap snapshot of approximately the same size, then streams JSON in chunks of at
+most 512 bytes; the ring is not copied onto the small HTTP task stack and no
+whole-response JSON string is allocated. Allocation failure returns 503, and
+send failures close the connection. Sample ages use rollover-safe uptime
+arithmetic and old samples are pruned on the main loop. Uninitialized, missing, NaN and infinite states become
 `null`. Sensor expiry stays with the parent configuration's existing filters
 (e.g. `timeout: 30s`); this component adds no independent sample-age policy.
 
@@ -128,7 +154,9 @@ the entity registry. Uninitialized, missing, NaN and infinite states become
 APIs were inspected against installed ESPHome 2026.10.2 and local ESP-IDF headers.
 Host checks cover schema/socket registration and codegen, C++ routing/serialization
 with API stand-ins (including send/shutdown/close ordering and close-queue
-failures), and the embedded JavaScript (threshold boundaries, missing
-and offline states, timeout and sequential polling). These are not a firmware or
+failures, history retention/wraparound, uptime rollover, allocation failures, and
+chunked-send failures), and the embedded JavaScript (threshold boundaries,
+missing and offline states, chart bounds/gaps, timeout and sequential polling).
+These are not a firmware or
 on-device integration test. The parent integration must build and check both
 listeners on hardware, including captive portal operation while Wi-Fi is down.