fix(fc3b): replace FC-3a KNOWN_PLATFORMS w/ registry; drop /api/sources/platforms

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

backend/app/services/source_service.py

@@ -9,11 +9,11 @@ from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..models import Artist, Source
+from .platforms import known_platform_keys
 
-# App-level allowlist (frozenset to make accidental mutation loud).
-KNOWN_PLATFORMS: frozenset[str] = frozenset({
-    "patreon", "fanbox", "subscribestar", "pixiv", "deviantart",
-})
+# Re-exported for back-compat with FC-3a; the registry in
+# services/platforms.py is the single source of truth.
+KNOWN_PLATFORMS = known_platform_keys()
 
 
 # --- Errors -----------------------------------------------------------------