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/chat_room/router.py

@@ -193,7 +193,7 @@ async def permanent_delete_room(
         "type": "system",
         "event": "room_deleted",
         "room_id": room_id,
-        "timestamp": datetime.utcnow().isoformat()
+        "timestamp": datetime.utcnow().isoformat() + "Z"
     })
 
     success, error = room_service.permanent_delete_room(db, room_id)
@@ -246,7 +246,7 @@ async def join_room(
             detail={
                 "message": "Already a member of this room",
                 "current_role": existing.role.value,
-                "added_at": existing.added_at.isoformat()
+                "added_at": existing.added_at.isoformat() + "Z"
             }
         )
 
@@ -505,12 +505,12 @@ async def add_lot(
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Room not found")
 
     # Get current lots or initialize empty list
-    current_lots = room.lots or []
+    current_lots = list(room.lots or [])  # Create a new list to ensure change detection
 
     # Prevent duplicates
     if request.lot not in current_lots:
         current_lots.append(request.lot)
-        room.lots = current_lots
+        room.lots = current_lots  # Assign new list triggers SQLAlchemy change detection
         room.last_updated_at = datetime.utcnow()
         db.commit()
         db.refresh(room)
@@ -532,11 +532,11 @@ async def remove_lot(
     if not room:
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Room not found")
 
-    current_lots = room.lots or []
+    current_lots = list(room.lots or [])  # Create a new list to ensure change detection
 
     if lot in current_lots:
         current_lots.remove(lot)
-        room.lots = current_lots
+        room.lots = current_lots  # Assign new list triggers SQLAlchemy change detection
         room.last_updated_at = datetime.utcnow()
         db.commit()
         db.refresh(room)