Delete hard-coded Repixelizer callback trust (deletion_line 2)

acceptsBackendCallback is now a pure per-app env allow-list lookup;
Repixelizer's callback must be configured via
GC_ACCESS_APP_REPIXELIZER_BACKEND_CALLBACK_URLS like every other app (R8:
operator sets this on the running legacy body before the new build starts).
Test config adds the allow-list entry so existing Repixelizer flows keep
passing, and a new negative test pins that the slug name alone buys nothing.

src/app.ts

@@ -508,15 +508,7 @@ function acceptsBackendCallback(config: HeimdallConfig, appSlug: AppSlug, handof
     return false;
   }
 
-  if ((config.appBackendCallbacks[appSlug] ?? []).includes(handoff.callbackUrl)) {
-    return true;
-  }
-
-  if (appSlug === "repixelizer") {
-    return handoff.callbackUrl === "https://repixelizer.gamecult.org/api/auth/heimdall/callback";
-  }
-
-  return false;
+  return (config.appBackendCallbacks[appSlug] ?? []).includes(handoff.callbackUrl);
 }
 
 async function maybeDeliverBackendHandoff(handoff: OAuthHandoff, payload: BackendHandoffPayload): Promise<void> {

tests/app.test.ts

@@ -32,6 +32,7 @@ function createTestConfig(): HeimdallConfig {
     },
     appBackendCallbacks: {
       bifrost: ["https://bifrost.gamecult.org/auth/heimdall/callback"],
+      repixelizer: ["https://repixelizer.gamecult.org/api/auth/heimdall/callback"],
     },
     storage: {
       backend: "memory",
@@ -898,6 +899,34 @@ describe("Heimdall service", () => {
     );
   });
 
+  it("no longer trusts Repixelizer's callback URL by slug name alone", async () => {
+    const baseConfig = createTestConfig();
+    const config = {
+      ...baseConfig,
+      appBackendCallbacks: {},
+    };
+    const app = await buildApp({ config });
+    apps.push(app);
+
+    const response = await app.inject({
+      method: "POST",
+      url: "/v1/oauth/discord/start",
+      payload: {
+        appSlug: "repixelizer",
+        mode: "sign_in",
+        returnTo: "https://repixelizer.gamecult.org/app/",
+        handoff: {
+          kind: "backend_callback",
+          attemptId: "attempt-123",
+          callbackUrl: "https://repixelizer.gamecult.org/api/auth/heimdall/callback",
+        },
+      },
+    });
+
+    expect(response.statusCode).toBe(400);
+    expect(response.json()).toEqual(expect.objectContaining({ error: "untrusted_backend_callback" }));
+  });
+
   it("resolves app-managed provider credentials without exposing refresh custody", async () => {
     const store = new InMemoryStore();
     const app = await buildApp({