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>

federation/entrypoint.sh

@@ -22,7 +22,7 @@ if [[ -n "${REDIS_URL:-}" && "${REDIS_URL}" != "no" ]]; then
   python3 -c "
 import redis, os
 r = redis.from_url(os.environ['REDIS_URL'])
-r.flushall()
+r.flushdb()
 print('Redis cache cleared.')
 " || echo "Redis flush failed (non-fatal), continuing..."
 fi