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/document-management/spec.md

@@ -193,6 +193,28 @@ The system SHALL warn users when deleting tasks with unresolved blockers.
 - **THEN** system auto-resolves all blockers with "task deleted" reason
 - **THEN** system proceeds with task deletion
 
+### Requirement: File MIME Type Validation
+The system SHALL validate file content type using magic bytes detection.
+
+#### Scenario: Valid file with matching extension
+- **WHEN** a user uploads a file
+- **AND** the detected MIME type matches the file extension
+- **THEN** the upload SHALL be accepted
+
+#### Scenario: Spoofed file extension rejected
+- **WHEN** a user uploads a file with extension `.jpg`
+- **AND** the actual content is detected as `application/x-executable`
+- **THEN** the upload SHALL be rejected with error "File type mismatch"
+
+#### Scenario: Unsupported MIME type rejected
+- **WHEN** a user uploads a file with an unsupported MIME type
+- **THEN** the upload SHALL be rejected with error "Unsupported file type"
+
+#### Scenario: MIME validation bypass for trusted sources
+- **WHEN** a file is uploaded from a trusted internal source
+- **AND** the system is configured to allow bypass
+- **THEN** MIME validation MAY be skipped
+
 ## Data Model
 
 ```