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>

openspec/specs/user-auth/spec.md

@@ -168,6 +168,35 @@ The system SHALL prevent file path traversal attacks by validating all file path
 - **THEN** system resolves path and verifies it is within storage directory
 - **THEN** system processes file operation normally
 
+### Requirement: JWT Secret Validation
+The system SHALL validate JWT secret key strength on startup.
+
+#### Scenario: Weak secret rejected
+- **WHEN** the configured JWT secret is less than 32 characters
+- **THEN** the system SHALL log a critical warning
+- **AND** optionally refuse to start in production mode
+
+#### Scenario: Low entropy secret warning
+- **WHEN** the JWT secret has low entropy (repeating patterns, common words)
+- **THEN** the system SHALL log a security warning
+
+### Requirement: CSRF Protection
+The system SHALL protect sensitive state-changing operations with CSRF tokens.
+
+#### Scenario: CSRF token required for password change
+- **WHEN** a user attempts to change their password
+- **AND** the request does not include a valid CSRF token
+- **THEN** the request SHALL be rejected with 403 Forbidden
+
+#### Scenario: CSRF token required for account deletion
+- **WHEN** a user attempts to delete their account or resources
+- **AND** the request does not include a valid CSRF token
+- **THEN** the request SHALL be rejected with 403 Forbidden
+
+#### Scenario: Valid CSRF token accepted
+- **WHEN** a state-changing request includes a valid CSRF token
+- **THEN** the request SHALL proceed normally
+
 ## Data Model
 
 ```