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/federation.py

@@ -23,8 +23,7 @@ class ActorProfile(Base):
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
     user_id: Mapped[int] = mapped_column(
-        Integer, ForeignKey("users.id", ondelete="CASCADE"),
-        unique=True, nullable=False,
+        Integer, unique=True, nullable=False,
     )
     preferred_username: Mapped[str] = mapped_column(String(64), unique=True, nullable=False)
     display_name: Mapped[str | None] = mapped_column(String(255), nullable=True)
@@ -36,7 +35,6 @@ class ActorProfile(Base):
     )
 
     # Relationships
-    user = relationship("User", backref="actor_profile", uselist=False, lazy="selectin")
     activities = relationship("APActivity", back_populates="actor_profile", lazy="dynamic")
     followers = relationship("APFollower", back_populates="actor_profile", lazy="dynamic")