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/main.py

@@ -3,12 +3,37 @@
 生產線異常即時反應系統 (Task Reporter)
 """
 import os
+import json
 from pathlib import Path
+from datetime import datetime
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.staticfiles import StaticFiles
-from fastapi.responses import FileResponse
+from fastapi.responses import FileResponse, JSONResponse
 from app.core.config import get_settings
+
+
+class UTCDateTimeEncoder(json.JSONEncoder):
+    """Custom JSON encoder that formats datetime with 'Z' suffix for UTC"""
+    def default(self, obj):
+        if isinstance(obj, datetime):
+            return obj.isoformat() + 'Z'
+        return super().default(obj)
+
+
+class UTCJSONResponse(JSONResponse):
+    """JSONResponse that uses UTCDateTimeEncoder"""
+    def render(self, content) -> bytes:
+        return json.dumps(
+            content,
+            ensure_ascii=False,
+            allow_nan=False,
+            indent=None,
+            separators=(",", ":"),
+            cls=UTCDateTimeEncoder,
+        ).encode("utf-8")
+
+
 from app.modules.auth import router as auth_router
 from app.modules.auth.users_router import router as users_router
 from app.modules.auth.middleware import auth_middleware
@@ -26,12 +51,13 @@ settings = get_settings()
 # Database tables are managed by Alembic migrations
 # Run: alembic upgrade head
 
-# Initialize FastAPI app
+# Initialize FastAPI app with custom JSON response for UTC datetime
 app = FastAPI(
     title="Task Reporter API",
     description="Production Line Incident Response System - 生產線異常即時反應系統",
     version="1.0.0",
     debug=settings.DEBUG,
+    default_response_class=UTCJSONResponse,
 )
 
 # CORS middleware - origins configured via CORS_ORIGINS environment variable