feat: implement security, error resilience, and query optimization proposals

Security Validation (enhance-security-validation):
- JWT secret validation with entropy checking and pattern detection
- CSRF protection middleware with token generation/validation
- Frontend CSRF token auto-injection for DELETE/PUT/PATCH requests
- MIME type validation with magic bytes detection for file uploads

Error Resilience (add-error-resilience):
- React ErrorBoundary component with fallback UI and retry functionality
- ErrorBoundaryWithI18n wrapper for internationalization support
- Page-level and section-level error boundaries in App.tsx

Query Performance (optimize-query-performance):
- Query monitoring utility with threshold warnings
- N+1 query fixes using joinedload/selectinload
- Optimized project members, tasks, and subtasks endpoints

Bug Fixes:
- WebSocket session management (P0): Return primitives instead of ORM objects
- LIKE query injection (P1): Escape special characters in search queries

Tests: 543 backend tests, 56 frontend tests passing

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

backend/app/schemas/auth.py

@@ -32,5 +32,11 @@ class TokenPayload(BaseModel):
     iat: int
 
 
+class CSRFTokenResponse(BaseModel):
+    """Response containing a CSRF token for state-changing operations."""
+    csrf_token: str = Field(..., description="CSRF token to include in X-CSRF-Token header")
+    expires_in: int = Field(default=3600, description="Token expiry time in seconds")
+
+
 # Update forward reference
 LoginResponse.model_rebuild()