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).

cmd/server/tests/playlist_collections_test.go

@@ -13,6 +13,8 @@ import (
 
 	"github.com/google/uuid"
 	"github.com/stretchr/testify/require"
+
+	"bookhoard/internal/database"
 )
 
 // A2c end-to-end: an ABS sidecar whose series array mixes a real series and
@@ -67,13 +69,14 @@ func TestPlaylistCollectionsEndToEnd(t *testing.T) {
 	require.NotEmpty(t, foundPath)
 
 	// (b) The playlist collection exists with provenance, dashboard pin,
-	// and the item at reading position 7.
+	// and the item at reading position 7. Owner-scoped read: collections
+	// RLS (2026-10-31) hides the row from the pool session's SeedIdentity.
 	adminUUIDStr := adminID.String()
 	var collID uuid.UUID
 	var collName, collDesc string
 	var showOnDashboard bool
 	var viewSettingsRaw []byte
-	require.NoError(t, setup.DBPool.QueryRow(context.Background(),
+	require.NoError(t, database.NewScopedDB(setup.DBPool).QueryRow(ownerSeedCtx(adminID),
 		`SELECT id, name, description, show_on_dashboard, view_settings
 		 FROM collections WHERE user_id = $1 AND name = 'The Tagged Saga (Reading Order)'`,
 		adminID).Scan(&collID, &collName, &collDesc, &showOnDashboard, &viewSettingsRaw))
@@ -85,7 +88,9 @@ func TestPlaylistCollectionsEndToEnd(t *testing.T) {
 
 	var position *int32
 	var itemCount int
-	require.NoError(t, setup.DBPool.QueryRow(context.Background(),
+	// Owner-scoped reads: collections RLS (2026-10-31) — items ride the
+	// parent's readability, so the pool session's SeedIdentity sees none.
+	require.NoError(t, database.NewScopedDB(setup.DBPool).QueryRow(ownerSeedCtx(adminID),
 		`SELECT ci.position, COUNT(*) OVER () FROM collection_items ci
 		 WHERE ci.collection_id = $1`, collID).Scan(&position, &itemCount))
 	require.Equal(t, 1, itemCount)
@@ -96,11 +101,11 @@ func TestPlaylistCollectionsEndToEnd(t *testing.T) {
 	// (c) Rescan: no duplicate collection, no duplicate membership.
 	scanTestFolder(t, setup, adminID, folder)
 	var collectionCount, membershipCount int
-	require.NoError(t, setup.DBPool.QueryRow(context.Background(),
+	require.NoError(t, database.NewScopedDB(setup.DBPool).QueryRow(ownerSeedCtx(adminID),
 		`SELECT COUNT(*) FROM collections WHERE user_id = $1 AND name = 'The Tagged Saga (Reading Order)'`,
 		adminID).Scan(&collectionCount))
 	require.Equal(t, 1, collectionCount)
-	require.NoError(t, setup.DBPool.QueryRow(context.Background(),
+	require.NoError(t, database.NewScopedDB(setup.DBPool).QueryRow(ownerSeedCtx(adminID),
 		`SELECT COUNT(*) FROM collection_items WHERE collection_id = $1`, collID).Scan(&membershipCount))
 	require.Equal(t, 1, membershipCount)
 
@@ -131,7 +136,9 @@ func TestPlaylistCollectionsEndToEnd(t *testing.T) {
 	// (e) hide_finished + Up Next + completion marker.
 	// Flip the collection's flag on, mark the single item finished for the
 	// admin, and the dashboard section becomes Complete with no items.
-	_, err = setup.DBPool.Exec(context.Background(),
+	// Owner-or-admin UPDATE policy: run as ADMIN (collections RLS,
+	// 2026-10-31).
+	adminScopeExec(t, setup, adminCtxValue(t, setup),
 		`UPDATE collections SET view_settings = view_settings || '{"hide_finished": true}'::jsonb WHERE id = $1`, collID)
 	require.NoError(t, err)
 	_, err = setup.DBPool.Exec(context.Background(),