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/migrations/0051_extension_points.sql

@@ -0,0 +1,40 @@
+-- §5.5 (v1.2 Hierarchies): extension-point markers — opt-in module polymorphism.
+--
+-- An extension point marks a module NAME at a node (app or group) as one
+-- descendants are *expected* to provide or override. Imports of an EP name
+-- resolve DYNAMICALLY against the inheriting app's view (the app can supply
+-- its own module), instead of the Phase 4b lexical default (sealed to the
+-- importing script's defining node). See docs §5.5.
+--
+-- This table holds only the MARKER (owner, name) — it is pure declaration,
+-- structurally identical to a `secrets` name. The optional DEFAULT BODY is
+-- just a co-located `kind = 'module'` script of the same name at this node
+-- (Phase 4b already stores/resolves/caches those); there is no body column
+-- here. So a marker is config, not code → ON DELETE CASCADE (unlike the
+-- module body's RESTRICT in 0050).
+--
+-- Ownership is polymorphic (mirrors vars/secrets/scripts): exactly one of
+-- (app_id, group_id) is set, with per-owner partial-unique LOWER(name)
+-- indexes for case-insensitive uniqueness.
+
+CREATE TABLE extension_points (
+    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    group_id    UUID REFERENCES groups(id) ON DELETE CASCADE,
+    app_id      UUID REFERENCES apps(id) ON DELETE CASCADE,
+    CONSTRAINT extension_points_owner_exactly_one
+        CHECK ((group_id IS NULL) <> (app_id IS NULL)),
+    name        TEXT NOT NULL,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- One marker per (owner, name), case-insensitive. Partial because the owner
+-- is split across two nullable columns.
+CREATE UNIQUE INDEX extension_points_group_uidx
+    ON extension_points (group_id, LOWER(name)) WHERE group_id IS NOT NULL;
+CREATE UNIQUE INDEX extension_points_app_uidx
+    ON extension_points (app_id, LOWER(name)) WHERE app_id IS NOT NULL;
+
+-- Lookup indexes for the resolver's chain join + list-by-owner.
+CREATE INDEX extension_points_group_id_idx ON extension_points (group_id) WHERE group_id IS NOT NULL;
+CREATE INDEX extension_points_app_id_idx ON extension_points (app_id) WHERE app_id IS NOT NULL;