Phase 9: OAuth & Account Switching — Google + Authentik, multi-account

Backend:
- OAuth service with pluggable provider architecture (Google + Authentik)
- Generic authorize/callback endpoints for any provider
- Authentik OIDC integration (configurable base URL)
- hashed_password made nullable for OAuth-only users
- Migration 009: nullable password column
- /auth/switch endpoint returns full AuthResponse for account switching
- OAuth-only users get clear error on password login attempt
- UserResponse includes oauth_provider + avatar_url

Frontend:
- OAuth buttons on login form (Google + Authentik)
- OAuth callback handler (/auth/callback route)
- Multi-account auth store (accounts array, addAccount, switchTo, removeAccount)
- Account switcher dropdown in header (hover to see other accounts)
- "Add another account" option
- English + Russian translations

Config:
- GOOGLE_CLIENT_ID/SECRET/REDIRECT_URI
- AUTHENTIK_CLIENT_ID/SECRET/BASE_URL/REDIRECT_URI

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/public/locales/en/translation.json

@@ -22,7 +22,12 @@
       "passwordMinLength": "Password must be at least 8 characters",
       "usernameFormat": "Username must be 3-50 characters (letters, numbers, _ or -)",
       "required": "This field is required"
-    }
+    },
+    "orDivider": "or continue with",
+    "oauthGoogle": "Sign in with Google",
+    "oauthAuthentik": "Sign in with Authentik",
+    "addAccount": "Add another account",
+    "switchAccount": "Switch account"
   },
   "layout": {
     "dashboard": "Dashboard",