Add on-demand Frigate+ API key validation (#24918)

* Add on-demand Frigate+ API key validation

The Frigate+ settings page reported the API key as validated whenever it
matched the expected format, without ever checking it with the server.

- Clarify the status text to say the key matches the expected format
- Add PlusApi.validate_key(), which requests a token from Frigate+ and
  reports the key as valid, invalid, unreachable, or not set; server
  errors and timeouts are not reported as an invalid key
- Add an admin-only POST /plus/validate endpoint and regenerate the API spec
- Add a "Validate with Frigate+" button that shows the result inline

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E4iDiZE2b3tVFVYyZrfZde

* Validate Frigate+ key asynchronously and keep cached token on bad replies

- Use aiohttp for the validation request so the endpoint awaits it
  instead of blocking a worker thread
- Only replace the cached token when the reply has a string accessToken
  and numeric expires; otherwise report unreachable so an unexpected
  response can't break later Frigate+ requests
- Test against a local aiohttp server, covering timeouts, connection
  failures, non-JSON replies and unusable token shapes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E4iDiZE2b3tVFVYyZrfZde

* Honor proxy settings when validating the Frigate+ key

aiohttp ignores HTTP(S)_PROXY unless trust_env is set, so validation
reported Frigate+ as unreachable on installs that need a proxy while
other Frigate+ requests still worked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E4iDiZE2b3tVFVYyZrfZde

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

web/public/locales/en/views/settings.json

@@ -1285,8 +1285,13 @@
     },
     "apiKey": {
       "title": "Frigate+ API Key",
-      "validated": "Frigate+ API key is detected and validated",
-      "notValidated": "Frigate+ API key is not detected or not validated",
+      "formatValid": "Frigate+ API key is detected and matches the expected format",
+      "formatInvalid": "Frigate+ API key is not detected or does not match the expected format",
+      "validate": "Validate with Frigate+",
+      "validating": "Validating…",
+      "serverValid": "Frigate+ accepted this API key",
+      "serverInvalid": "Frigate+ rejected this API key",
+      "serverUnreachable": "Unable to reach Frigate+ to validate this API key",
       "desc": "The Frigate+ API key enables integration with the Frigate+ service.",
       "plusLink": "Read more about Frigate+"
     },