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/db/session.py

@@ -35,6 +35,72 @@ async def get_session():
         await sess.close()
 
 
+# ---------------------------------------------------------------------------
+# Cross-domain sessions — account + federation
+#
+# Initially DATABASE_URL_ACCOUNT / DATABASE_URL_FEDERATION point to the same
+# DB as DATABASE_URL (zero behaviour change).  When per-domain DBs are ready,
+# switch the env vars to the new connection strings.
+# ---------------------------------------------------------------------------
+
+DATABASE_URL_ACCOUNT = (
+    os.getenv("DATABASE_URL_ACCOUNT") or DATABASE_URL
+)
+
+DATABASE_URL_FEDERATION = (
+    os.getenv("DATABASE_URL_FEDERATION") or DATABASE_URL
+)
+
+# Engines are created lazily — only allocate a pool if the URL differs
+_account_engine = (
+    _engine if DATABASE_URL_ACCOUNT == DATABASE_URL
+    else create_async_engine(
+        DATABASE_URL_ACCOUNT,
+        future=True, echo=False, pool_pre_ping=True,
+        pool_size=3, max_overflow=5,
+    )
+)
+_AccountSession = async_sessionmaker(
+    bind=_account_engine,
+    class_=AsyncSession,
+    expire_on_commit=False,
+)
+
+_federation_engine = (
+    _engine if DATABASE_URL_FEDERATION == DATABASE_URL
+    else create_async_engine(
+        DATABASE_URL_FEDERATION,
+        future=True, echo=False, pool_pre_ping=True,
+        pool_size=3, max_overflow=5,
+    )
+)
+_FederationSession = async_sessionmaker(
+    bind=_federation_engine,
+    class_=AsyncSession,
+    expire_on_commit=False,
+)
+
+
+@asynccontextmanager
+async def get_account_session():
+    """Session targeting the account database (users, grants, oauth codes)."""
+    sess = _AccountSession()
+    try:
+        yield sess
+    finally:
+        await sess.close()
+
+
+@asynccontextmanager
+async def get_federation_session():
+    """Session targeting the federation database (ap_activities, etc.)."""
+    sess = _FederationSession()
+    try:
+        yield sess
+    finally:
+        await sess.close()
+
+
 def register_db(app: Quart):
 
     @app.before_request