Fix MCP update_memory metadata handling and database sync

Fixed three critical bugs in MCP update_memory tool:

1. SQLAlchemy mutation tracking: Create new dict object instead of
   modifying and reassigning same reference (change detection failed)

2. Empty metadata clearing: Filter system fields first, then update
   with request metadata. Empty {} now correctly clears custom fields
   while preserving system fields (source_app, mcp_client, etc.)

3. Database synchronization: Prepare merged metadata once and send
   same data to both mem0 (Qdrant) and PostgreSQL. Previously they
   received different metadata causing divergence.

Behavior:
- metadata=None: PostgreSQL unchanged, Qdrant preserved by mem0 core
- metadata={}: Clear custom fields, keep system fields in both DBs
- metadata={"key": "val"}: System + custom fields in both DBs

Pattern follows add_memories: prepare combined metadata once, send to
both databases for perfect synchronization.

Author: frederikb96

openmemory/api/app/mcp_server.py

@@ -20,6 +20,7 @@
 import json
 import logging
 import uuid
+from typing import Optional
 
 from app.database import SessionLocal
 from app.models import Memory, MemoryAccessLog, MemoryState, MemoryStatusHistory
@@ -289,7 +290,7 @@ async def list_memories() -> str:
 
 
 @mcp.tool(description="Update a memory's content and custom metadata")
-async def update_memory(memory_id: str, text: str, metadata: dict = {}) -> str:
+async def update_memory(memory_id: str, text: str, metadata: Optional[dict] = None) -> str:
     uid = user_id_var.get(None)
     client_name = client_name_var.get(None)
     if not uid:
@@ -318,17 +319,33 @@ async def update_memory(memory_id: str, text: str, metadata: dict = {}) -> str:
             if not check_memory_access_permissions(db, memory, app.id):
                 return "Error: No permission to update this memory"
 
-            # Update in mem0 using public API (handles metadata protection automatically)
-            response = memory_client.update(memory_id=memory_id, data=text, metadata=metadata)
+            # Prepare metadata update (merge with system fields)
+            metadata_for_vector_store = None
+            if metadata is not None:
+                # System fields that should never be overwritten
+                SYSTEM_FIELDS = ['user_id', 'agent_id', 'run_id', 'actor_id', 'role', 'source_app', 'mcp_client']
 
-            # Update in database
-            memory.content = text
-
-            # Merge custom metadata (preserve existing, update/add new)
-            if metadata:
+                # Start with only system fields from existing metadata
                 existing_metadata = memory.metadata_ or {}
-                existing_metadata.update(metadata)
-                memory.metadata_ = existing_metadata
+                new_metadata = {k: v for k, v in existing_metadata.items() if k in SYSTEM_FIELDS}
+
+                # Add all custom fields from request (empty request = clear custom metadata)
+                new_metadata.update(metadata)
+
+                metadata_for_vector_store = new_metadata
+
+            # Update in mem0 using public API (with merged metadata)
+            # If metadata_for_vector_store is None, mem0 will preserve existing metadata in Qdrant
+            response = await memory_client.update(
+                memory_id=memory_id,
+                data=text,
+                metadata=metadata_for_vector_store
+            )
+
+            # Update in database (same merged metadata)
+            memory.content = text
+            if metadata is not None:
+                memory.metadata_ = metadata_for_vector_store
 
             # Create history entry
             history = MemoryStatusHistory(