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>

internal/db/settings.sql.go

@@ -10,6 +10,22 @@ import (
 	"database/sql"
 )
 
+const deleteIntegrationsGoogle = `-- name: DeleteIntegrationsGoogle :execrows
+DELETE FROM integrations_google
+WHERE id = 1 AND refresh_token_ct = ?
+`
+
+// Deletes the row only while it still holds refresh_token_ct, so a
+// connection made after the caller read the row is kept. Returns the number
+// of rows deleted.
+func (q *Queries) DeleteIntegrationsGoogle(ctx context.Context, refreshTokenCt []byte) (int64, error) {
+	result, err := q.db.ExecContext(ctx, deleteIntegrationsGoogle, refreshTokenCt)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected()
+}
+
 const getAISettings = `-- name: GetAISettings :one
 SELECT id, provider, base_url, model, temperature, system_prompt, api_key_ct, api_key_last4, key_verified_at, created_at, updated_at FROM ai_settings
 WHERE id = 1