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).

AGENTS.md

@@ -145,6 +145,17 @@ every user sees every library unless an admin hides one) is enforced by
   new pages/controls go into `tests/ui/COVERAGE.md`). Item-scoped
   gates may answer 404 as well as 403 (row invisible vs Go-level
   check — contract_pins 2026-10-29).
+- **Collections RLS (2026-10-31)**: `collections` and
+  `collection_items` are governed too — predicates
+  `app_can_read_collection` / `app_can_write_collection`, PURE over
+  their arguments (never query `collections`, so no policy recursion
+  under FORCE RLS). Reads: owner-or-admin; writes: owner, admin escape
+  for SYSTEM collections only. Future sharing = one EXISTS branch
+  inside the predicate functions, no policy migration. Registration
+  seeding declares the new user's identity (`rls.WithUser` in
+  `CreateDefaultCollectionsForUser` — the JWT does not exist yet);
+  test-harness writes to non-system collections must run as the row's
+  OWNER (`scopeExecAs`/`ownerSeedCtx`, not the admin handle).
 
 History: 2026-10-13 (`431fec0`) the series/media listing surfaces
 leaked hidden libraries; **2026-10-29 enforcement moved to RLS** — the