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>

_config/init-databases.sql

@@ -0,0 +1,12 @@
+-- Per-domain databases for the coop stack.
+-- Run once on fresh deployments (not needed for existing single-DB setups
+-- that use the split-databases.sh migration script instead).
+--
+-- Usage:  psql -U postgres -f init-databases.sql
+
+CREATE DATABASE db_account;
+CREATE DATABASE db_blog;
+CREATE DATABASE db_market;
+CREATE DATABASE db_cart;
+CREATE DATABASE db_events;
+CREATE DATABASE db_federation;