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/seedsample/main.go

@@ -243,7 +243,7 @@ func must(err error) {
 // fatal logs through the application JSON logger and exits -- the
 // structured replacement for log.Fatal.
 func fatal(msg string, err error) {
-	applog.App("main", "fatal").Error(msg, "error", err)
+	applog.App().Error(msg, "error", err)
 	os.Exit(1)
 }
 
@@ -255,7 +255,7 @@ func mustFindKindID(ctx context.Context, db *store.DB, userID int64, name string
 			return k.ID
 		}
 	}
-	applog.App("main", "mustFindKindID").Error("no workout kind with that name for the seeded user (did ProvisionUser seed the taxonomy?)", "kind", name)
+	applog.App().Error("no workout kind with that name for the seeded user (did ProvisionUser seed the taxonomy?)", "kind", name)
 	os.Exit(1)
 	return 0
 }