feat: multi-Oracle server support with runtime switching

Complete implementation of multi-server Oracle database support:

Backend:
- Multi-pool Oracle with lazy loading per server
- Email-to-server cache for automatic server discovery
- JWT tokens include server_id claim
- /auth/check-identity and /auth/check-email endpoints
- /auth/my-servers endpoint for listing user's accessible servers
- Server switch with password re-authentication

Frontend:
- New ServerSelector component for header dropdown
- Multi-step login flow (identity → server → password)
- Server switching from header with password modal
- Mobile drawer menu with server selection
- Dark mode support for all new components
- URL bookmark support with ?server= query param

Scripts:
- Unified start.sh replacing start-prod.sh/start-test.sh
- Unified ssh-tunnel.sh with multi-server support
- Updated status.sh for new architecture

Tests:
- E2E tests for multi-server and single-server login flows
- Backend unit tests for all new endpoints
- Oracle multi-pool integration tests

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

shared/routes/system.py

@@ -13,6 +13,12 @@ from pydantic import BaseModel
 from shared.auth.dependencies import get_current_user, CurrentUser
 
 
+class AuthModeResponse(BaseModel):
+    """Response for auth mode endpoint."""
+    mode: str  # "single-server" or "multi-server"
+    supports_email_login: bool  # True if email-based login is available
+
+
 class LogEntry(BaseModel):
     """Single log entry."""
     line: str
@@ -36,6 +42,36 @@ def create_system_router() -> APIRouter:
     """
     router = APIRouter()
 
+    @router.get("/auth-mode", response_model=AuthModeResponse)
+    async def get_auth_mode():
+        """
+        Get the authentication mode configuration.
+
+        This is a PUBLIC endpoint (no auth required) that tells the frontend
+        whether to use the email-based multi-server login flow or the classic
+        username/password single-server flow.
+
+        Returns:
+            - mode: "single-server" for legacy config, "multi-server" for ORACLE_SERVERS
+            - supports_email_login: True only in multi-server mode with email cache
+        """
+        from backend.config import settings
+
+        servers = settings.get_oracle_servers()
+
+        # Multi-server mode: 2+ servers configured via ORACLE_SERVERS
+        if servers and len(servers) > 1:
+            return AuthModeResponse(
+                mode="multi-server",
+                supports_email_login=True
+            )
+
+        # Single-server mode: legacy config or single ORACLE_SERVERS entry
+        return AuthModeResponse(
+            mode="single-server",
+            supports_email_login=False
+        )
+
     def get_logs_path() -> Path:
         """Get logs directory path based on environment."""
         # Windows production: C:\inetpub\wwwroot\roa2web\logs