refactor(store,tools): migrate IDs from UUID to bigserial int64

All internal entity lookups now use bigserial primary keys (int64) while
GUIDs are retained for external/public identification. Updated store
functions (TouchProject, UpdateThoughtMetadata, AddThoughtAttachment) to
query by id instead of guid, added GetThoughtByID, changed semanticSearch
and all tool helpers to use *int64 project IDs, and updated retry/backfill
workers to use int64 thought IDs throughout.

internal/tools/update.go

@@ -83,7 +83,7 @@ func (t *UpdateTool) Handle(ctx context.Context, _ *mcp.CallToolRequest, in Upda
 		if err != nil {
 			return nil, UpdateOutput{}, err
 		}
-		projectID = &project.ID
+		projectID = &project.NumericID
 	}
 
 	updated, err := t.store.UpdateThought(ctx, id, content, embedding, embeddingModel, mergedMetadata, projectID)