feat(collections): RLS ownership enforcement at the DB layer

collections + collection_items join the RLS-governed tables, same
doctrine as the media_items family lens (2026-10-29). Policies route
through two PURE predicate functions (app_can_read_collection /
app_can_write_collection) that never query collections — no policy
recursion under FORCE RLS — so a future sharing model is one EXISTS
branch inside the functions, not a schema re-touch. Reads: owner or
admin; writes: owner, admin escape for SYSTEM collections only.

Why: the write path had no app-level ownership gate on Update/Delete/
AddBooks — any authenticated user could mutate another user's
collection by id. The DB is now the enforcement of record; existing
app-level gates (match-mode, reorder, view-settings) stay as
defense-in-depth.

Behavior changes (contract_pins 2026-10-31):
- cross-user GET detail: 200 -> 404 (row invisible)
- cross-user PUT update: silent success -> 404 (ErrNoRows mapping;
  service wrap fixed %v -> %w so errors.Is sees pgx.ErrNoRows)
- cross-user POST books: silent success -> 404
- cross-user DELETE: 204, now a true DB no-op
- anonymous sees nothing; admin persona unchanged

Bootstrap: registration seeds system collections BEFORE the JWT
exists — CreateDefaultCollectionsForUser declares the new user's
identity (rls.WithUser), the same fix mirrored in the test harness
(createDefaultCollectionsForUser owner-scoped; collection sweeps run
under the owner's identity).

Tests: new TestCollectionsRLS pins the cross-user matrix end-to-end
(404 detail/update/add/view-settings, 204 no-op delete, admin escape,
owner positive control, registration seeding under RLS). Harness
fixture writes to non-system collections now run as the row's OWNER
(scopeExecAs/ownerSeedCtx) — the admin write-escape covers system
rows only, and bare-ctx pool reads of collections see nothing.
Filter/playlist/audiobook/scan-state fixtures converted. Full
container suite green (159s).

database/schema/schema.sql

@@ -2180,3 +2180,117 @@ CREATE POLICY p_podcast_feeds_delete ON podcast_feeds
 -- policy's lookup table, and a user must always see their own
 -- subscription rows (subscribe -> visibility is a single INSERT).
 -- ============================================================
+-- ── collections + collection_items (2026-10-31): ownership at the DB ──
+-- layer, same doctrine as media_items/podcast_feeds above. The
+-- predicates live in FUNCTIONS (not inline) so a future sharing model
+-- edits exactly one place: add the share-table EXISTS branch inside
+-- app_can_read_collection / app_can_write_collection. The functions are
+-- PURE over their arguments — they never query collections — so the
+-- policies cannot recurse, FORCE RLS safe. Reads are authenticated-
+-- allow (household-readable: any signed-in identity, SeedIdentity
+-- harness included); WRITES are owner-or-admin — that is the
+-- enforcement of record for Update/Delete/AddBooks, which have no
+-- app-level gate.
+-- ===========================================================
+
+CREATE OR REPLACE FUNCTION app_is_admin() RETURNS BOOLEAN
+LANGUAGE sql STABLE SECURITY DEFINER SET search_path = pg_catalog, public AS $$
+  SELECT EXISTS (
+    SELECT 1 FROM users u
+    WHERE u.id = NULLIF(current_setting('app.user_id', true), '')::uuid
+      AND u.role = 'admin'
+  );
+$$;
+
+CREATE OR REPLACE FUNCTION app_can_read_collection(
+  p_collection_id UUID, p_owner UUID
+) RETURNS BOOLEAN
+LANGUAGE sql STABLE SECURITY DEFINER SET search_path = pg_catalog, public AS $$
+  SELECT NULLIF(current_setting('app.user_id', true), '') IS NOT NULL
+     AND (
+       p_owner = current_setting('app.user_id', true)::uuid
+       OR app_is_admin()
+       -- FUTURE SHARING slot: OR EXISTS (
+       --   SELECT 1 FROM collection_shares cs
+       --   WHERE cs.collection_id = p_collection_id
+       --     AND cs.shared_with_user_id = current_setting('app.user_id', true)::uuid)
+     );
+$$;
+
+CREATE OR REPLACE FUNCTION app_can_write_collection(
+  p_collection_id UUID, p_owner UUID, p_system BOOLEAN
+) RETURNS BOOLEAN
+LANGUAGE sql STABLE SECURITY DEFINER SET search_path = pg_catalog, public AS $$
+  SELECT NULLIF(current_setting('app.user_id', true), '') IS NOT NULL
+     AND (
+       p_owner = current_setting('app.user_id', true)::uuid
+       OR (COALESCE(p_system, false) AND app_is_admin())
+       -- FUTURE SHARING slot: OR EXISTS (
+       --   SELECT 1 FROM collection_shares cs
+       --   WHERE cs.collection_id = p_collection_id
+       --     AND cs.shared_with_user_id = current_setting('app.user_id', true)::uuid
+       --     AND cs.permission IN ('edit'))
+     );
+$$;
+
+ALTER TABLE collections ENABLE ROW LEVEL SECURITY;
+ALTER TABLE collections FORCE ROW LEVEL SECURITY;
+
+DROP POLICY IF EXISTS p_collections_select ON collections;
+CREATE POLICY p_collections_select ON collections
+  FOR SELECT USING (app_can_read_collection(id, user_id));
+DROP POLICY IF EXISTS p_collections_insert ON collections;
+CREATE POLICY p_collections_insert ON collections
+  FOR INSERT WITH CHECK (
+    user_id = NULLIF(current_setting('app.user_id', true), '')::uuid
+    OR app_is_admin()
+  );
+DROP POLICY IF EXISTS p_collections_update ON collections;
+CREATE POLICY p_collections_update ON collections
+  FOR UPDATE
+  USING (app_can_write_collection(id, user_id, is_system_collection))
+  WITH CHECK (app_can_write_collection(id, user_id, is_system_collection));
+DROP POLICY IF EXISTS p_collections_delete ON collections;
+CREATE POLICY p_collections_delete ON collections
+  FOR DELETE USING (app_can_write_collection(id, user_id, is_system_collection));
+
+ALTER TABLE collection_items ENABLE ROW LEVEL SECURITY;
+ALTER TABLE collection_items FORCE ROW LEVEL SECURITY;
+
+-- Items ride their PARENT's accessibility: the collections SELECT
+-- policy auto-applies inside these EXISTS subqueries.
+DROP POLICY IF EXISTS p_collection_items_select ON collection_items;
+CREATE POLICY p_collection_items_select ON collection_items
+  FOR SELECT USING (EXISTS (
+    SELECT 1 FROM collections c
+    WHERE c.id = collection_items.collection_id
+  ));
+DROP POLICY IF EXISTS p_collection_items_insert ON collection_items;
+CREATE POLICY p_collection_items_insert ON collection_items
+  FOR INSERT WITH CHECK (EXISTS (
+    SELECT 1 FROM collections c
+    WHERE c.id = collection_items.collection_id
+      AND app_can_write_collection(c.id, c.user_id, c.is_system_collection)
+  ));
+DROP POLICY IF EXISTS p_collection_items_update ON collection_items;
+CREATE POLICY p_collection_items_update ON collection_items
+  FOR UPDATE
+  USING (EXISTS (
+    SELECT 1 FROM collections c
+    WHERE c.id = collection_items.collection_id
+      AND app_can_write_collection(c.id, c.user_id, c.is_system_collection)
+  ))
+  WITH CHECK (EXISTS (
+    SELECT 1 FROM collections c
+    WHERE c.id = collection_items.collection_id
+      AND app_can_write_collection(c.id, c.user_id, c.is_system_collection)
+  ));
+DROP POLICY IF EXISTS p_collection_items_delete ON collection_items;
+CREATE POLICY p_collection_items_delete ON collection_items
+  FOR DELETE USING (EXISTS (
+    SELECT 1 FROM collections c
+    WHERE c.id = collection_items.collection_id
+      AND app_can_write_collection(c.id, c.user_id, c.is_system_collection)
+  ));
+
+