feat(modules): extension-point marker schema + repo (§5.5 C1)

An extension point (§5.5) marks a module name a node offers for descendants
to provide/override — resolved dynamically against the inheriting app rather
than lexically sealed. Lay the storage:

- migration 0051: owner-polymorphic `extension_points(id, group_id?, app_id?,
  name)` marker table — exactly-one CHECK + per-owner partial-unique LOWER(name)
  indexes + lookup indexes (mirrors 0050). ON DELETE CASCADE (a marker is
  config, not code). No default-body column — the optional default body is a
  co-located kind=module script (Phase 4b stores/resolves/caches those).
- extension_point_repo: `list_for_owner` + idempotent `insert_extension_point_tx`
  / `delete_extension_point_tx`, keyed by the shared `ScriptOwner` (mirrors the
  var/secret tx-fn style).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/lib.rs

@@ -44,6 +44,7 @@ pub mod docs_repo;
 pub mod docs_service;
 pub mod email_inbound_api;
 pub mod email_service;
+pub mod extension_point_repo;
 pub mod files_api;
 pub mod files_repo;
 pub mod files_service;