fix: handle FileNotFoundError gracefully during sync

When a file exists in the database but is missing from the filesystem,
the sync worker now treats this as a deletion instead of crashing.

The sync_file() method catches FileNotFoundError specifically and calls
handle_delete() to clean up the orphaned database record. This prevents
the sync from failing on database/filesystem inconsistencies that can
occur due to race conditions, manual file deletions, or cloud storage
caching issues.

Includes a test to verify the graceful handling behavior.

Fixes #386

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/sync/sync_service.py

@@ -610,6 +610,16 @@ class SyncService:
                 )
             return entity, checksum
 
+        except FileNotFoundError:
+            # File exists in database but not on filesystem
+            # This indicates a database/filesystem inconsistency - treat as deletion
+            logger.warning(
+                f"File not found during sync, treating as deletion: path={path}. "
+                "This may indicate a race condition or manual file deletion."
+            )
+            await self.handle_delete(path)
+            return None, None
+
         except Exception as e:
             # Check if this is a fatal error (or caused by one)
             # Fatal errors like project deletion should terminate sync immediately