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/ghost_content.py

@@ -94,7 +94,7 @@ class Post(Base):
     deleted_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
 
     user_id: Mapped[Optional[int]] = mapped_column(
-        Integer, ForeignKey("users.id", ondelete="SET NULL"), index=True
+        Integer, index=True
     )
     publish_requested: Mapped[bool] = mapped_column(Boolean(), default=False, server_default="false", nullable=False)
 
@@ -111,9 +111,6 @@ class Post(Base):
     primary_tag: Mapped[Optional[Tag]] = relationship(
         "Tag", foreign_keys=[primary_tag_id]
     )
-    user: Mapped[Optional["User"]] = relationship(
-        "User", foreign_keys=[user_id]
-    )
 
     # AUTHORS RELATIONSHIP (many-to-many via post_authors)
     authors: Mapped[List["Author"]] = relationship(
@@ -205,7 +202,7 @@ class PostLike(Base):
     __tablename__ = "post_likes"
 
     id = Column(Integer, primary_key=True, autoincrement=True)
-    user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    user_id = Column(Integer, nullable=False)
     post_id: Mapped[int] = mapped_column(ForeignKey("posts.id", ondelete="CASCADE"), nullable=False)
 
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
@@ -213,4 +210,3 @@ class PostLike(Base):
     deleted_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True))
 
     post: Mapped["Post"] = relationship("Post", back_populates="likes", foreign_keys=[post_id])
-    user = relationship("User", back_populates="liked_posts")