feat: manga collections (0.17.0)

User-owned named lists of mangas with an add-to-collection modal on
the manga page and dedicated /collections and /collections/:id pages.

- Schema (0010): `collections` (per-user case-insensitive name
  uniqueness) + `collection_mangas` join with cascade FKs.
- Endpoints: full CRUD on `/v1/collections`, idempotent add/remove
  for `/v1/collections/:id/mangas`, and `/v1/mangas/:id/my-collections`
  for the modal's pre-checked state. Owner-mismatch surfaces as 404
  (not 403) so the API doesn't disclose collection existence to
  non-owners; the frontend funnels 401 to /login. Three-state PATCH
  via a new shared `domain::patch::Patch<T>` lets clients distinguish
  "leave alone", "clear", and "set" for description.
- Frontend: reusable `Modal` component (focus trap, opt-in
  backdrop close, ESC) and `AddToCollectionModal` with optimistic
  toggling that's race-safe under fast clicks. /collections page
  renders cover-collage cards; /collections/:id is editable with
  per-card remove. Top nav gets a Collections link.

155 backend tests (incl. 21 collection tests covering ownership,
idempotence, sample-cover enrichment, three-state PATCH, FK race);
88 frontend tests; svelte-check clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/migrations/0010_collections.sql

@@ -0,0 +1,31 @@
+-- User-owned manga collections. Each user can curate any number of
+-- named lists (e.g., "Favorites", "Reading list"); mangas can belong
+-- to many collections of many users without restriction.
+
+CREATE TABLE collections (
+    id          uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id     uuid NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    name        text NOT NULL,
+    description text,
+    created_at  timestamptz NOT NULL DEFAULT now(),
+    updated_at  timestamptz NOT NULL DEFAULT now()
+);
+
+-- Per-user case-insensitive name uniqueness so "Favorites" and
+-- "favorites" don't both end up in someone's sidebar.
+CREATE UNIQUE INDEX collections_user_name_lower_uniq
+    ON collections (user_id, lower(name));
+
+CREATE INDEX collections_user_idx ON collections (user_id, created_at DESC);
+
+CREATE TABLE collection_mangas (
+    collection_id uuid NOT NULL REFERENCES collections(id) ON DELETE CASCADE,
+    manga_id      uuid NOT NULL REFERENCES mangas(id) ON DELETE CASCADE,
+    added_at      timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (collection_id, manga_id)
+);
+
+-- Reverse lookup: which collections contain this manga? Used by the
+-- "Add to collection" modal to pre-check the boxes for the user's
+-- collections this manga is already in.
+CREATE INDEX collection_mangas_manga_idx ON collection_mangas (manga_id);