refactor(log): split location into package/file/class/method, auto-derived

Per the log schema: 'package' is the Go package (absent on
Python-emitted lines), 'file' the Go/Python source basename, 'class' the
Go receiver or Python class (omitted when there is none -- free
functions no longer masquerade under a package-as-class), 'method' the
emitting function. applog.App() now takes no arguments and derives all
of it from runtime.Caller, so labels can never drift from the code; the
manual http/wrapper emitters and forwarded wrapper.py lines (file=
wrapper.py, no package) carry the same fields.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/api/setup.go

@@ -195,10 +195,10 @@ func (s *Server) handleSetupComplete(w http.ResponseWriter, r *http.Request) {
 		// on, and os.Rename refuses to replace a non-empty directory. The
 		// just-validated setup session must win, so clear the target first.
 		if err := os.RemoveAll(newDir); err != nil {
-			applog.App("api.Server", "handleSetupComplete").Error("remove stale token store dir", "user_id", userID, "error", err)
+			applog.App().Error("remove stale token store dir", "user_id", userID, "error", err)
 		}
 		if err := os.Rename(oldDir, newDir); err != nil && !os.IsNotExist(err) {
-			applog.App("api.Server", "handleSetupComplete").Error("rename setup token store dir", "user_id", userID, "error", err)
+			applog.App().Error("rename setup token store dir", "user_id", userID, "error", err)
 		}
 	}