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/alembic/env.py

@@ -0,0 +1,17 @@
+from alembic import context
+from shared.db.alembic_env import run_alembic
+
+MODELS = [
+    "shared.models.sx_pub",
+]
+
+TABLES = frozenset({
+    "sx_pub_actor",
+    "sx_pub_collections",
+    "sx_pub_documents",
+    "sx_pub_activities",
+    "sx_pub_followers",
+    "sx_pub_following",
+})
+
+run_alembic(context.config, MODELS, TABLES)