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/cmd/dumpschema/main.go

@@ -93,12 +93,12 @@ func main() {
 
 	outPath := "../docs/DATABASE.md"
 	must(os.WriteFile(outPath, []byte(b.String()), 0644))
-	applog.App("main", "main").Info("wrote schema doc", "path", outPath)
+	applog.App().Info("wrote schema doc", "path", outPath)
 }
 
 func must(err error) {
 	if err != nil {
-		applog.App("main", "must").Error("dumpschema failed", "error", err)
+		applog.App().Error("dumpschema failed", "error", err)
 		os.Exit(1)
 	}
 }