fix(storage): emit origin-relative object URLs

Build signed object URLs as path-only (/api/objects/...) instead of
prefixing APP_BASE_URL, so media loads from whatever host or reverse
proxy served the app. Remove the now-unused base URL from the signer and
drop the APP_BASE_URL / Server.BaseURL config.

backend/internal/e2e/sqlite_test.go

@@ -65,7 +65,7 @@ func openStack(t *testing.T, path, blobRoot string) (*stack, func()) {
 	}
 	repos := store.NewSQLite(database)
 	hub := sse.NewHub(1)
-	signer, err := storage.NewSigner("http://app.test", "secret", time.Hour, 10*time.Minute)
+	signer, err := storage.NewSigner("secret", time.Hour, 10*time.Minute)
 	if err != nil {
 		t.Fatalf("signer: %v", err)
 	}