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/log/log_test.go

@@ -60,13 +60,19 @@ func TestApp_TagsMandatoryFields(t *testing.T) {
 	slog.SetDefault(NewLogger("info", &buf))
 	defer slog.SetDefault(prev)
 
-	App("api.Server", "handleThing").Info("did it", "extra", 1)
+	App().Info("did it", "extra", 1)
 
 	var decoded map[string]any
 	if err := json.Unmarshal(buf.Bytes(), &decoded); err != nil {
 		t.Fatalf("output not JSON: %v", err)
 	}
-	if decoded["type"] != "app" || decoded["class"] != "api.Server" || decoded["method"] != "handleThing" {
-		t.Errorf("mandatory fields = %+v, want type=app class=api.Server method=handleThing", decoded)
+	if decoded["type"] != "app" || decoded["package"] != "log" || decoded["file"] != "log_test.go" {
+		t.Errorf("fields = %+v, want type=app package=log (import-path element) file=log_test.go", decoded)
+	}
+	if decoded["method"] != "TestApp_TagsMandatoryFields" {
+		t.Errorf("method = %v, want the emitting function name", decoded["method"])
+	}
+	if _, hasClass := decoded["class"]; hasClass {
+		t.Errorf("class should be omitted for a free function, got %v", decoded["class"])
 	}
 }