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/models/user.py

@@ -30,13 +30,8 @@ class User(Base):
     labels = relationship("GhostLabel", secondary="user_labels", back_populates="users", lazy="selectin")
     subscriptions = relationship("GhostSubscription", back_populates="user", cascade="all, delete-orphan", lazy="selectin")
 
-    liked_products = relationship("ProductLike", back_populates="user", cascade="all, delete-orphan")
-    liked_posts = relationship("PostLike", back_populates="user", cascade="all, delete-orphan")
-    cart_items = relationship(
-      "CartItem",
-      back_populates="user",
-      cascade="all, delete-orphan",
-    )
+    # Cross-domain reverse relationships removed (liked_products, liked_posts,
+    # cart_items) — those tables live in different domain DBs
 
     __table_args__ = (
         Index("ix_user_email", "email", unique=True),