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>

docs/static/frigate-api.yaml

@@ -3148,6 +3148,25 @@ paths:
         - frigateUserAuth: []
       x-required-role: any
       description: '**Access:** Any authenticated user.'
+  /plus/validate:
+    post:
+      tags:
+        - App
+      summary: Validate Plus Key
+      description: |-
+        **Access:** Admin role required.
+
+        Validate the configured Frigate+ API key with the Frigate+ server.
+      operationId: validate_plus_key_plus_validate_post
+      responses:
+        '200':
+          description: Successful Response
+          content:
+            application/json:
+              schema: {}
+      security:
+        - frigateAdminAuth: []
+      x-required-role: admin
   /recognized_license_plates:
     get:
       tags: