feat(I6): FE/BE enum contract test — pin JS mirrors to backend canon

The plain-JS frontend re-declares two backend enum value-sets by hand:
download-event statuses (downloadStatus.js) and platform keys
(platformColor.js). With no TS codegen, drift is silent — the same class
as the last_verified_at field mismatch.

tests/test_fe_be_contract.py parses both JS mirrors and asserts they equal
the backend canon (downloads._ALLOWED_STATUSES, platforms.known_platform_keys())
so a change on either side fails CI on whichever moved. Backend is the
source of truth. Documented the invariant in both JS file headers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/utils/downloadStatus.js

@@ -1,7 +1,8 @@
 // Single source of truth for the download-event status enum -> display
 // metadata (label/color/icon). Mirrors the backend ENUM
 // (pending|running|ok|error|skipped). Used by the stat chips, the filter
-// dropdown, and the event rows so they never drift apart.
+// dropdown, and the event rows so they never drift apart. The value set is
+// pinned against backend _ALLOWED_STATUSES by tests/test_fe_be_contract.py.
 
 export const DOWNLOAD_STATUSES = [
   { value: 'pending', label: 'Queued',    color: 'grey',    icon: 'mdi-clock-outline' },

frontend/src/utils/platformColor.js

@@ -1,7 +1,9 @@
 // Single source of truth for platform → color + icon mapping. Used by
 // PlatformChip and any other GS-style platform-tagged surface. The six
 // platforms FC supports map 1:1 to the GS palette; unknown platforms fall
-// back to grey + mdi-web. Operator-confirmed scope 2026-05-27.
+// back to grey + mdi-web. Operator-confirmed scope 2026-05-27. The ICONS key
+// set is pinned against backend known_platform_keys() by
+// tests/test_fe_be_contract.py.
 
 const ICONS = {
   patreon:       'mdi-patreon',

tests/test_fe_be_contract.py

@@ -0,0 +1,44 @@
+"""FE/BE enum contract — guards against the field/enum drift class.
+
+FabledCurator's frontend is plain JS (no TS codegen), so the frontend
+re-declares a few backend enum value-sets by hand:
+
+  - download-event statuses  →  frontend/src/utils/downloadStatus.js
+  - platform keys            →  frontend/src/utils/platformColor.js
+
+If either side edits its list without the other, the UI silently
+mislabels rows or drops a platform (the `last_verified_at`-style drift).
+These tests parse the JS mirrors and assert they equal the backend's
+canonical definitions, so any divergence fails CI on whichever side moved.
+The backend definitions are the source of truth; update the JS to match.
+"""
+
+import re
+from pathlib import Path
+
+from backend.app.api.downloads import _ALLOWED_STATUSES
+from backend.app.services.platforms import known_platform_keys
+
+_FRONTEND = Path(__file__).resolve().parents[1] / "frontend" / "src" / "utils"
+
+
+def _read(name: str) -> str:
+    return (_FRONTEND / name).read_text(encoding="utf-8")
+
+
+def test_download_status_values_match_backend() -> None:
+    # downloadStatus.js: each entry is `{ value: 'pending', ... }`
+    fe_values = set(re.findall(r"value:\s*'([a-z]+)'", _read("downloadStatus.js")))
+    assert fe_values == set(_ALLOWED_STATUSES), (
+        "downloadStatus.js DOWNLOAD_STATUSES is out of sync with backend "
+        "downloads._ALLOWED_STATUSES"
+    )
+
+
+def test_platform_keys_match_backend() -> None:
+    # platformColor.js ICONS: each entry is `patreon: 'mdi-patreon',`
+    fe_keys = set(re.findall(r"^\s*(\w+):\s*'mdi-", _read("platformColor.js"), re.M))
+    assert fe_keys == set(known_platform_keys()), (
+        "platformColor.js ICONS is out of sync with backend "
+        "platforms.known_platform_keys()"
+    )