feat(auth+admin): Sign in with Google (OAuth) + Integrations config panel

Backend (identity-svc):
- oauth_config table (mig 22) + OAuthConfig entity
- OAuthService: admin config CRUD + Google authorization-code flow (build consent
  URL, exchange code, fetch userinfo, find/create RegisterMode.Google user, issue
  session via AuthService.IssueOAuthSessionAsync)
- AuthController: GET /v1/auth/google/{start,callback} (public); tokens handed to
  frontend via URL fragment
- AdminController: GET/PUT /v1/admin/oauth/{provider} (admin, secret masked)

Frontend:
- "ورود با گوگل" button on /auth → identity start endpoint
- /auth/callback reads fragment tokens → /api/auth/oauth-session sets httpOnly cookies
- /admin/integrations: Google client_id/secret/redirect_uri + enable, with setup guide
- nav + fa/en labels

Client ID/Secret are configured entirely in the admin panel — no redeploy needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

messages/en.json

@@ -332,7 +332,8 @@
       "music": "Music",
       "homeEvents": "Home Events",
       "comments": "Comments",
-      "routes": "Internal Routes"
+      "routes": "Internal Routes",
+      "integrations": "Integrations"
     },
     "appAdminNodesPage": {
       "title": "Render Nodes",
@@ -444,7 +445,9 @@
       "passwordLabel": "Password",
       "forgotPassword": "Forgot password?",
       "createAccount": "Create Account",
-      "legalNotice": "By continuing, you agree to our <terms>Terms</terms> and <privacy>Privacy Policy</privacy>."
+      "legalNotice": "By continuing, you agree to our <terms>Terms</terms> and <privacy>Privacy Policy</privacy>.",
+      "orContinueWith": "or continue with",
+      "continueWithGoogle": "Continue with Google"
     },
     "componentsAuthSupabaseSetupNotice": {
       "title": "Supabase not configured",

messages/fa.json

@@ -332,7 +332,8 @@
       "music": "موسیقی",
       "homeEvents": "رویدادهای صفحه اصلی",
       "comments": "نظرات",
-      "routes": "مسیرهای داخلی"
+      "routes": "مسیرهای داخلی",
+      "integrations": "یکپارچه‌سازی‌ها"
     },
     "appAdminNodesPage": {
       "title": "نودهای رندر",
@@ -444,7 +445,9 @@
       "passwordLabel": "رمز عبور",
       "forgotPassword": "رمز عبور را فراموش کرده‌اید؟",
       "createAccount": "ساخت حساب",
-      "legalNotice": "با ادامه دادن، با <terms>قوانین</terms> و <privacy>سیاست حفظ حریم خصوصی</privacy> ما موافقت می‌کنید."
+      "legalNotice": "با ادامه دادن، با <terms>قوانین</terms> و <privacy>سیاست حفظ حریم خصوصی</privacy> ما موافقت می‌کنید.",
+      "orContinueWith": "یا ادامه با",
+      "continueWithGoogle": "ورود با گوگل"
     },
     "componentsAuthSupabaseSetupNotice": {
       "title": "Supabase پیکربندی نشده است",