Split databases and Redis — prepare infrastructure for per-domain isolation

Redis: per-app DB index (0-5) with shared auth DB 15 for SSO keys;
flushdb replaces flushall so deploys don't wipe cross-app auth state.

Postgres: drop 13 cross-domain FK constraints (migration v2t0p8q9r0),
remove dead ORM relationships, add explicit joins for 4 live ones.
Multi-engine sessions (account + federation) ready for per-domain DBs
via DATABASE_URL_ACCOUNT / DATABASE_URL_FEDERATION env vars.

All URLs initially point to the same appdb — zero behaviour change
until split-databases.sh is run to migrate data to per-domain DBs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/infrastructure/oauth.py

@@ -78,12 +78,14 @@ def create_oauth_blueprint(app_name: str) -> Blueprint:
             qsession["_pnone_at"] = _time.time()
             device_id = g.device_id
             if device_id:
-                from shared.browser.app.redis_cacher import get_redis
-                _redis = get_redis()
-                if _redis:
-                    await _redis.set(
+                from shared.infrastructure.auth_redis import get_auth_redis
+                try:
+                    _auth_r = await get_auth_redis()
+                    await _auth_r.set(
                         f"prompt:{app_name}:{device_id}", b"none", ex=300
                     )
+                except Exception:
+                    pass
             return redirect(next_url)
 
         code = request.args.get("code")