Add Google Calendar OAuth connect and disconnect

GET /api/v1/integrations/google/connect sends the browser to Google's
consent screen (calendar.events and calendar.calendarlist.readonly,
access_type=offline, prompt=consent, PKCE S256) with a state carried in a
signed __Host-mealquest_google flow cookie bound to the session.
GET /api/v1/integrations/google/callback checks the state and session,
exchanges the code, refuses a response without a refresh token
(reconnect_required) or without both scopes, seals the refresh token with
secretbox into integrations_google and sets connected_at.
DELETE /api/v1/integrations/google revokes the token at Google and clears
it; a revoke failure keeps the connection so it can be retried.

- internal/gcal: the connect service; Google endpoints are overridable and
  internal/gcal/gcaltest fakes the token and revoke endpoints.
- internal/auth: the login cookie becomes the exported FlowCookie both
  flows use (same wire format); CheckRedirect and ExchangeError are
  exported for reuse.
- internal/config: MEALQUEST_GOOGLE_CLIENT_ID and
  MEALQUEST_GOOGLE_CLIENT_SECRET_FILE, required at boot.
- DeleteIntegrationsGoogle query (sqlc regenerated); no migration, the
  integrations_google table already exists.
- deploy: compose mounts google_client_secret; .env.example and RUNBOOK
  document the Google OAuth client, its redirect URI and rotation.

Maestro task 1e9d7850-3a09-4572-b8a1-cb18cd4774cb.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/compose.yaml

@@ -32,10 +32,12 @@ services:
       MEALQUEST_DATA_DIR: /data
       MEALQUEST_SECRET_KEY_FILE: /run/secrets/secret_key
       MEALQUEST_OIDC_CLIENT_SECRET_FILE: /run/secrets/oidc_client_secret
+      MEALQUEST_GOOGLE_CLIENT_SECRET_FILE: /run/secrets/google_client_secret
       MEALQUEST_SEARXNG_URL: http://searxng:8080
     secrets:
       - secret_key
       - oidc_client_secret
+      - google_client_secret
     volumes:
       - mealquest-data:/data
     ports:
@@ -128,6 +130,8 @@ secrets:
     file: ./secrets/secret_key
   oidc_client_secret:
     file: ./secrets/oidc_client_secret
+  google_client_secret:
+    file: ./secrets/google_client_secret
 
 volumes:
   mealquest-data: