feat: remove app name and logo customization feature

Users can no longer customize the app name or logo. The branding API
endpoints, permission, frontend UI, env vars (APP_NAME, MAX_LOGO_SIZE_KB),
and all related tests are removed. Includes a migration to clean up
branding data from existing databases.

tests/integration/api-key-scoping.test.ts

@@ -77,7 +77,7 @@ describe("API key permission scoping", () => {
       method: "PUT",
       url: "/api/v1/settings",
       headers: { authorization: `Bearer ${apiKey}` },
-      payload: { appName: "hacked" },
+      payload: { testSetting: "hacked" },
     });
     expect(writeRes.statusCode).toBe(403);
   });