feat: Complete file upload V3 integration

Fix 4: Integrated Gradio file uploads with V3 architecture

What Changed:
- File uploads now go to S3 (sessions/{id}/files/{file_id}_{name})
- Creates records in MongoDB 'files' collection
- Generates presigned URLs (7-day expiry)
- Stores file metadata (file_id, size, mime_type, s3_key)
- Adds file_id reference to session
- Includes file_data in system messages
- Graceful fallback if S3 upload fails

Files Collection Now Populated:
- file_id, session_id, file_name
- s3_bucket, s3_key
- presigned_url, presigned_expires_at
- file_size, mime_type, uploaded_at

Benefits:
- Gradio UI and chat/unified API now in sync
- Files stored permanently in S3
- File access via presigned URLs
- Consistent behavior across interfaces

All 5 critical V3 fixes now complete!

app.py

@@ -752,7 +752,13 @@ def create_new_session():
 
 
 def handle_file_upload(file_path, session_id):
-    """Handle file upload"""
+    """
+    Handle file upload with V3 architecture integration
+    - Uploads file to S3
+    - Creates record in files collection
+    - Generates presigned URL
+    - Stores file metadata in MongoDB
+    """
     if not file_path:
         return None, json.dumps({
             "status": "error",
@@ -764,31 +770,128 @@ def handle_file_upload(file_path, session_id):
     
     file_name = os.path.basename(file_path)
     
-    # Update session
-    session_manager.update_session(session_id, {
-        "current_file": file_path,
-        "state": ConversationState.INITIAL
-    })
-    
-    # Add system message
-    session_manager.add_message(
-        session_id,
-        "system",
-        f"File uploaded: {file_name}"
-    )
-    
-    status = {
-        "status": "success",
-        "message": f"File '{file_name}' uploaded successfully",
-        "file_info": {
-            "name": file_name,
-            "path": file_path,
-            "size_bytes": os.path.getsize(file_path) if os.path.exists(file_path) else 0
-        },
-        "next_action": "💬 Now tell me what you'd like to do with this document"
-    }
-    
-    return file_path, json.dumps(status, indent=2), session_id
+    try:
+        # V3: Upload to S3 and create file record
+        from services.s3_manager import get_s3_manager
+        from services.schemas import FileSchema, schema_to_dict
+        from datetime import datetime
+        
+        s3 = get_s3_manager()
+        file_id = str(uuid.uuid4())
+        file_size = os.path.getsize(file_path) if os.path.exists(file_path) else 0
+        
+        # Determine MIME type
+        mime_type = "application/pdf"
+        if file_path.lower().endswith(('.png', '.jpg', '.jpeg')):
+            mime_type = "image/jpeg"
+        elif file_path.lower().endswith('.gif'):
+            mime_type = "image/gif"
+        
+        # Upload to S3
+        s3_key = f"sessions/{session_id}/files/{file_id}_{file_name}"
+        with open(file_path, 'rb') as f:
+            s3_result = s3.upload_file(
+                key=s3_key,
+                file_obj=f,
+                content_type=mime_type,
+                add_prefix=False
+            )
+        
+        # Generate presigned URL (7-day max)
+        presigned = s3.generate_presigned_url(
+            s3_key,
+            expires_in=604800,  # 7 days
+            add_prefix=False
+        )
+        
+        # Create file record in MongoDB
+        mongodb_uri = os.getenv("MONGODB_URI")
+        mongodb_db = os.getenv("MONGODB_DB", "masterllm")
+        from pymongo import MongoClient
+        
+        mongo_client = MongoClient(mongodb_uri)
+        files_collection = mongo_client[mongodb_db]["files"]
+        
+        file_record = FileSchema(
+            file_id=file_id,
+            session_id=session_id,
+            uploaded_at=datetime.utcnow(),
+            file_name=file_name,
+            file_size=file_size,
+            mime_type=mime_type,
+            s3_bucket=s3.bucket_name,
+            s3_key=s3_key,
+            presigned_url=presigned["presigned_url"],
+            presigned_expires_at=presigned["presigned_expires_at"]
+        )
+        
+        files_collection.insert_one(schema_to_dict(file_record))
+        
+        print(f"✅ File uploaded to S3 and recorded: {file_id}")
+        
+        # Update session with file reference
+        session_manager.update_session(session_id, {
+            "current_file": file_path,  # Keep local path for backward compatibility
+            "current_file_id": file_id,  # V3: file_id reference
+            "state": ConversationState.INITIAL
+        })
+        
+        # Add system message with file info
+        session_manager.add_message(
+            session_id,
+            "system",
+            f"File uploaded: {file_name}",
+            file_data={"file_id": file_id, "file_name": file_name}
+        )
+        
+        status = {
+            "status": "success",
+            "message": f"File '{file_name}' uploaded successfully",
+            "file_info": {
+                "file_id": file_id,
+                "name": file_name,
+                "size_bytes": file_size,
+                "s3_key": s3_key,
+                "presigned_url": presigned["presigned_url"],
+                "expires_at": presigned["presigned_expires_at"]
+            },
+            "next_action": "💬 Now tell me what you'd like to do with this document"
+        }
+        
+        return file_path, json.dumps(status, indent=2), session_id
+        
+    except Exception as e:
+        # Fallback to old behavior if V3 integration fails
+        print(f"⚠️ File upload V3 integration failed: {e}")
+        print(f"   Falling back to local file path")
+        
+        # Update session with local path only
+        session_manager.update_session(session_id, {
+            "current_file": file_path,
+            "state": ConversationState.INITIAL
+        })
+        
+        # Add system message
+        session_manager.add_message(
+            session_id,
+            "system",
+            f"File uploaded: {file_name}"
+        )
+        
+        status = {
+            "status": "partial_success",
+            "message": f"File '{file_name}' uploaded locally (S3 upload failed)",
+            "file_info": {
+                "name": file_name,
+                "path": file_path,
+                "size_bytes": os.path.getsize(file_path) if os.path.exists(file_path) else 0
+            },
+            "warning": str(e),
+            "next_action": "💬 Now tell me what you'd like to do with this document"
+        }
+        
+        return file_path, json.dumps(status, indent=2), session_id
+
 
 
 def format_chat_history(history, new_user_msg, new_assistant_msg):