sx-pub Phase 1: DB schema, IPFS wiring, actor + webfinger + collections + status endpoints

Foundation for the sx-pub federated publishing protocol:

- 6 SQLAlchemy models: actor, collections, documents, activities, followers, following
- Conditional DB enablement in sx_docs (DATABASE_URL present → enable DB)
- Python IO helpers: get_or_create_actor (auto-generates RSA keypair),
  list_collections, check_status, seed_default_collections
- 4 defhandler endpoints returning text/sx (no JSON):
  /pub/actor, /pub/webfinger, /pub/collections, /pub/status
- Alembic migration infrastructure for sx service
- Docker compose: DB + pgbouncer + IPFS + env vars

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.dev-pub.yml

@@ -11,6 +11,8 @@ services:
       SECRET_KEY: "${SECRET_KEY:-pub-dev-secret}"
       REDIS_URL: redis://redis:6379/0
       DATABASE_URL: postgresql+asyncpg://postgres:change-me@pgbouncer:5432/sx_pub
+      ALEMBIC_DATABASE_URL: postgresql+psycopg://postgres:change-me@db:5432/sx_pub
+      SX_PUB_DOMAIN: pub.sx-web.org
       WORKERS: "1"
       ENVIRONMENT: development
       RELOAD: "true"