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>

docker-compose.yml

@@ -11,6 +11,8 @@ x-app-common: &app-common
 
 x-app-env: &app-env
   DATABASE_URL: postgresql+asyncpg://postgres:change-me@db:5432/appdb
+  DATABASE_URL_ACCOUNT: postgresql+asyncpg://postgres:change-me@db:5432/appdb
+  DATABASE_URL_FEDERATION: postgresql+asyncpg://postgres:change-me@db:5432/appdb
   ALEMBIC_DATABASE_URL: postgresql+psycopg://postgres:change-me@db:5432/appdb
   SMTP_HOST: ${SMTP_HOST}
   SMTP_PORT: ${SMTP_PORT}
@@ -23,7 +25,7 @@ x-app-env: &app-env
   GHOST_CONTENT_API_KEY: ${GHOST_CONTENT_API_KEY}
   GHOST_WEBHOOK_SECRET: ${GHOST_WEBHOOK_SECRET}
   GHOST_ADMIN_API_KEY: ${GHOST_ADMIN_API_KEY}
-  REDIS_URL: redis://redis:6379
+  REDIS_AUTH_URL: redis://redis:6379/15
   SECRET_KEY: ${SECRET_KEY}
   SUMUP_API_KEY: ${SUMUP_API_KEY}
   APP_URL_BLOG: https://blog.rose-ash.com
@@ -56,6 +58,7 @@ services:
       dockerfile: blog/Dockerfile
     environment:
       <<: *app-env
+      REDIS_URL: redis://redis:6379/0
       DATABASE_HOST: db
       DATABASE_PORT: "5432"
       RUN_MIGRATIONS: "true"
@@ -71,6 +74,7 @@ services:
       - /root/rose-ash/_snapshot:/app/_snapshot
     environment:
       <<: *app-env
+      REDIS_URL: redis://redis:6379/1
       DATABASE_HOST: db
       DATABASE_PORT: "5432"
 
@@ -82,6 +86,7 @@ services:
       dockerfile: cart/Dockerfile
     environment:
       <<: *app-env
+      REDIS_URL: redis://redis:6379/2
       DATABASE_HOST: db
       DATABASE_PORT: "5432"
 
@@ -93,6 +98,7 @@ services:
       dockerfile: events/Dockerfile
     environment:
       <<: *app-env
+      REDIS_URL: redis://redis:6379/3
       DATABASE_HOST: db
       DATABASE_PORT: "5432"
 
@@ -104,6 +110,7 @@ services:
       dockerfile: federation/Dockerfile
     environment:
       <<: *app-env
+      REDIS_URL: redis://redis:6379/4
       DATABASE_HOST: db
       DATABASE_PORT: "5432"
 
@@ -115,6 +122,7 @@ services:
       dockerfile: account/Dockerfile
     environment:
       <<: *app-env
+      REDIS_URL: redis://redis:6379/5
       DATABASE_HOST: db
       DATABASE_PORT: "5432"