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/alembic/versions/v2t0p8q9r0_drop_cross_domain_fks.py

@@ -0,0 +1,59 @@
+"""Drop cross-domain foreign key constraints.
+
+Columns and indexes remain — only the FK constraints are removed.
+This prepares for per-domain databases where cross-DB FKs can't exist.
+
+Revision ID: v2t0p8q9r0
+Revises: u1s9o5p7q8
+"""
+from alembic import op
+
+revision = "v2t0p8q9r0"
+down_revision = "u1s9o5p7q8"
+
+
+def upgrade() -> None:
+    # blog → account
+    op.drop_constraint("posts_user_id_fkey", "posts", type_="foreignkey")
+    op.drop_constraint("post_likes_user_id_fkey", "post_likes", type_="foreignkey")
+
+    # market → account
+    op.drop_constraint("product_likes_user_id_fkey", "product_likes", type_="foreignkey")
+
+    # cart → account
+    op.drop_constraint("cart_items_user_id_fkey", "cart_items", type_="foreignkey")
+    op.drop_constraint("orders_user_id_fkey", "orders", type_="foreignkey")
+
+    # cart → market
+    op.drop_constraint("cart_items_product_id_fkey", "cart_items", type_="foreignkey")
+    op.drop_constraint("cart_items_market_place_id_fkey", "cart_items", type_="foreignkey")
+    op.drop_constraint("order_items_product_id_fkey", "order_items", type_="foreignkey")
+
+    # cart → events
+    op.drop_constraint("orders_page_config_id_fkey", "orders", type_="foreignkey")
+
+    # events → account
+    op.drop_constraint("calendar_entries_user_id_fkey", "calendar_entries", type_="foreignkey")
+    op.drop_constraint("tickets_user_id_fkey", "tickets", type_="foreignkey")
+
+    # federation → account
+    op.drop_constraint("ap_actor_profiles_user_id_fkey", "ap_actor_profiles", type_="foreignkey")
+
+    # shared (blog-internal but cross-concern)
+    op.drop_constraint("menu_items_post_id_fkey", "menu_items", type_="foreignkey")
+
+
+def downgrade() -> None:
+    op.create_foreign_key("posts_user_id_fkey", "posts", "users", ["user_id"], ["id"], ondelete="SET NULL")
+    op.create_foreign_key("post_likes_user_id_fkey", "post_likes", "users", ["user_id"], ["id"], ondelete="CASCADE")
+    op.create_foreign_key("product_likes_user_id_fkey", "product_likes", "users", ["user_id"], ["id"], ondelete="CASCADE")
+    op.create_foreign_key("cart_items_user_id_fkey", "cart_items", "users", ["user_id"], ["id"], ondelete="CASCADE")
+    op.create_foreign_key("cart_items_product_id_fkey", "cart_items", "products", ["product_id"], ["id"], ondelete="CASCADE")
+    op.create_foreign_key("cart_items_market_place_id_fkey", "cart_items", "market_places", ["market_place_id"], ["id"], ondelete="SET NULL")
+    op.create_foreign_key("orders_user_id_fkey", "orders", "users", ["user_id"], ["id"])
+    op.create_foreign_key("orders_page_config_id_fkey", "orders", "page_configs", ["page_config_id"], ["id"], ondelete="SET NULL")
+    op.create_foreign_key("order_items_product_id_fkey", "order_items", "products", ["product_id"], ["id"])
+    op.create_foreign_key("calendar_entries_user_id_fkey", "calendar_entries", "users", ["user_id"], ["id"])
+    op.create_foreign_key("tickets_user_id_fkey", "tickets", "users", ["user_id"], ["id"])
+    op.create_foreign_key("ap_actor_profiles_user_id_fkey", "ap_actor_profiles", "users", ["user_id"], ["id"], ondelete="CASCADE")
+    op.create_foreign_key("menu_items_post_id_fkey", "menu_items", "posts", ["post_id"], ["id"], ondelete="CASCADE")