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/schemas.py

@@ -2,7 +2,7 @@
 
 Request and response models for API endpoints
 """
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, ConfigDict, field_serializer
 from typing import Optional, List
 from datetime import datetime
 from enum import Enum
@@ -98,8 +98,14 @@ class MemberResponse(BaseModel):
     added_at: datetime
     removed_at: Optional[datetime] = None
 
-    class Config:
-        from_attributes = True
+    model_config = ConfigDict(from_attributes=True)
+
+    @field_serializer("added_at", "removed_at")
+    def serialize_datetime(self, dt: Optional[datetime]) -> Optional[str]:
+        """Serialize datetime with 'Z' suffix to indicate UTC"""
+        if dt is None:
+            return None
+        return dt.isoformat() + "Z"
 
 
 class RoomResponse(BaseModel):
@@ -127,8 +133,17 @@ class RoomResponse(BaseModel):
     is_member: bool = False
     is_admin_view: bool = False
 
-    class Config:
-        from_attributes = True
+    model_config = ConfigDict(from_attributes=True)
+
+    @field_serializer(
+        "created_at", "resolved_at", "archived_at",
+        "last_activity_at", "last_updated_at", "ownership_transferred_at"
+    )
+    def serialize_datetime(self, dt: Optional[datetime]) -> Optional[str]:
+        """Serialize datetime with 'Z' suffix to indicate UTC"""
+        if dt is None:
+            return None
+        return dt.isoformat() + "Z"
 
 
 class RoomListResponse(BaseModel):