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>

sx/app.py

@@ -62,8 +62,10 @@ def create_app() -> "Quart":
 
     extra_kw = {}
     if SX_STANDALONE:
-        extra_kw["no_db"] = True
         extra_kw["no_oauth"] = True
+        # Enable DB if DATABASE_URL is set (needed for sx-pub)
+        if not os.getenv("DATABASE_URL"):
+            extra_kw["no_db"] = True
 
     app = create_base_app(
         "sx",