feat: implement 5 QA-driven security and quality proposals

Implemented proposals from comprehensive QA review:

1. extend-csrf-protection
   - Add POST to CSRF protected methods in frontend
   - Global CSRF middleware for all state-changing operations
   - Update tests with CSRF token fixtures

2. tighten-cors-websocket-security
   - Replace wildcard CORS with explicit method/header lists
   - Disable query parameter auth in production (code 4002)
   - Add per-user WebSocket connection limit (max 5, code 4005)

3. shorten-jwt-expiry
   - Reduce JWT expiry from 7 days to 60 minutes
   - Add refresh token support with 7-day expiry
   - Implement token rotation on refresh
   - Frontend auto-refresh when token near expiry (<5 min)

4. fix-frontend-quality
   - Add React.lazy() code splitting for all pages
   - Fix useCallback dependency arrays (Dashboard, Comments)
   - Add localStorage data validation in AuthContext
   - Complete i18n for AttachmentUpload component

5. enhance-backend-validation
   - Add SecurityAuditMiddleware for access denied logging
   - Add ErrorSanitizerMiddleware for production error messages
   - Protect /health/detailed with admin authentication
   - Add input length validation (comment 5000, desc 10000)

All 521 backend tests passing. Frontend builds successfully.

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

backend/app/schemas/auth.py

@@ -9,10 +9,25 @@ class LoginRequest(BaseModel):
 
 class LoginResponse(BaseModel):
     access_token: str
+    refresh_token: str
     token_type: str = "bearer"
+    expires_in: int = Field(default=3600, description="Access token expiry in seconds")
     user: "UserInfo"
 
 
+class RefreshTokenRequest(BaseModel):
+    """Request body for refresh token endpoint."""
+    refresh_token: str = Field(..., description="The refresh token to use for obtaining a new access token")
+
+
+class RefreshTokenResponse(BaseModel):
+    """Response for refresh token endpoint."""
+    access_token: str
+    refresh_token: str  # New refresh token (rotation)
+    token_type: str = "bearer"
+    expires_in: int = Field(default=3600, description="Access token expiry in seconds")
+
+
 class UserInfo(BaseModel):
     id: str
     email: str