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/report_generation/schemas.py

@@ -2,7 +2,7 @@
 
 Request and response models for the report generation 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
@@ -45,8 +45,12 @@ class ReportStatusResponse(BaseModel):
     prompt_tokens: Optional[int] = None
     completion_tokens: Optional[int] = None
 
-    class Config:
-        from_attributes = True
+    model_config = ConfigDict(from_attributes=True)
+
+    @field_serializer("generated_at")
+    def serialize_datetime(self, dt: datetime) -> str:
+        """Serialize datetime with 'Z' suffix to indicate UTC"""
+        return dt.isoformat() + "Z"
 
 
 class ReportListItem(BaseModel):
@@ -57,8 +61,12 @@ class ReportListItem(BaseModel):
     status: ReportStatus
     report_title: Optional[str] = None
 
-    class Config:
-        from_attributes = True
+    model_config = ConfigDict(from_attributes=True)
+
+    @field_serializer("generated_at")
+    def serialize_datetime(self, dt: datetime) -> str:
+        """Serialize datetime with 'Z' suffix to indicate UTC"""
+        return dt.isoformat() + "Z"
 
 
 class ReportListResponse(BaseModel):