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/projects.go

@@ -79,7 +79,7 @@ func (t *ProjectsTool) SetActive(ctx context.Context, req *mcp.CallToolRequest,
 		return nil, SetActiveProjectOutput{}, err
 	}
 	t.sessions.Set(sid, project.ID)
-	_ = t.store.TouchProject(ctx, project.ID)
+	_ = t.store.TouchProject(ctx, project.NumericID)
 	return nil, SetActiveProjectOutput{Project: *project}, nil
 }