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>

backend/app/schemas/auth.py

@@ -29,6 +29,8 @@ class UserResponse(BaseModel):
     role: str
     is_active: bool
     max_chats: int
+    oauth_provider: str | None = None
+    avatar_url: str | None = None
     created_at: datetime
 
     model_config = {"from_attributes": True}