integrate sub2api as upstream for auth/keys/usage via FastAPI BFF

Preserve local user table for superDream-specific features while syncing
user lifecycle, API key CRUD and usage queries through sub2api. Admin token
handles reads and user lifecycle; per-user tokens (Fernet-encrypted in DB)
handle key writes that admin endpoints do not expose.

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

app/integrations/sub2api/__init__.py

@@ -0,0 +1,18 @@
+from app.integrations.sub2api.client import (
+    Sub2APIError,
+    Sub2APIReauthRequired,
+    Sub2APITransportError,
+    get_client,
+    close_client,
+)
+from app.integrations.sub2api import admin, user
+
+__all__ = [
+    "Sub2APIError",
+    "Sub2APIReauthRequired",
+    "Sub2APITransportError",
+    "get_client",
+    "close_client",
+    "admin",
+    "user",
+]