feat: Improve file display, timezone handling, and LOT management

Changes:
- Fix datetime serialization with UTC 'Z' suffix for correct timezone display
- Add PDF upload support with extension fallback for MIME detection
- Fix LOT add/remove by creating new list for SQLAlchemy JSON change detection
- Add file message components (FileMessage, ImageLightbox, UploadPreview)
- Add multi-file upload support with progress tracking
- Link uploaded files to chat messages via message_id
- Include file attachments in AI report generation
- Update specs for file-storage, realtime-messaging, and ai-report-generation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/modules/realtime/websocket_manager.py

@@ -14,7 +14,8 @@ settings = get_settings()
 def json_serializer(obj: Any) -> str:
     """Custom JSON serializer for objects not serializable by default json code"""
     if isinstance(obj, datetime):
-        return obj.isoformat()
+        # Append 'Z' to indicate UTC so JavaScript parses it correctly
+        return obj.isoformat() + 'Z'
     raise TypeError(f"Object of type {type(obj).__name__} is not JSON serializable")