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/file_storage/validators.py

@@ -1,7 +1,8 @@
 """File validation utilities"""
 import magic
+import os
 from fastapi import UploadFile, HTTPException
-from typing import Set
+from typing import Set, Dict
 import logging
 
 from app.core.config import get_settings
@@ -17,7 +18,15 @@ IMAGE_TYPES: Set[str] = {
 }
 
 DOCUMENT_TYPES: Set[str] = {
-    "application/pdf"
+    "application/pdf",
+    "application/x-pdf",  # Some systems detect PDF as x-pdf
+}
+
+# Extensions that can be accepted even if MIME detection fails
+EXTENSION_FALLBACK: Dict[str, str] = {
+    ".pdf": "application/pdf",
+    ".doc": "application/msword",
+    ".docx": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
 }
 
 LOG_TYPES: Set[str] = {
@@ -67,6 +76,17 @@ def validate_file_type(file: UploadFile, allowed_types: Set[str]) -> str:
     detected_mime = detect_mime_type(header)
 
     if detected_mime not in allowed_types:
+        # Try extension fallback for known safe file types
+        filename = file.filename or ""
+        _, ext = os.path.splitext(filename.lower())
+
+        if ext in EXTENSION_FALLBACK:
+            logger.info(
+                f"MIME detection returned {detected_mime} for {filename}, "
+                f"using extension fallback: {EXTENSION_FALLBACK[ext]}"
+            )
+            return EXTENSION_FALLBACK[ext]
+
         raise HTTPException(
             status_code=400,
             detail=f"File type not allowed: {detected_mime}. Allowed types: {', '.join(allowed_types)}"
@@ -115,9 +135,12 @@ def get_file_type_and_limits(mime_type: str) -> tuple[str, int]:
     Raises:
         HTTPException if MIME type not recognized
     """
+    # Include extension fallback types as documents
+    document_types_extended = DOCUMENT_TYPES | set(EXTENSION_FALLBACK.values())
+
     if mime_type in IMAGE_TYPES:
         return ("image", settings.get_image_max_size_bytes())
-    elif mime_type in DOCUMENT_TYPES:
+    elif mime_type in document_types_extended:
         return ("document", settings.get_document_max_size_bytes())
     elif mime_type in LOG_TYPES:
         return ("log", settings.get_log_max_size_bytes())