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>

src/app/[locale]/admin/layout.tsx

@@ -31,6 +31,7 @@ export default async function AdminLayout({
     { href: "/admin/files", label: t("media") },
     { href: "/admin/ai", label: t("aiContent") },
     { href: "/admin/messaging", label: t("messaging") },
+    { href: "/admin/integrations", label: t("integrations") },
     { href: "/admin/marketing", label: t("marketing") },
     { href: "/admin/crm", label: t("crm") },
     { href: "/admin/users", label: t("users") },