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/src/api/mod.rs

@@ -2,6 +2,7 @@ pub mod auth;
 pub mod authors;
 pub mod bookmarks;
 pub mod chapters;
+pub mod collections;
 pub mod files;
 pub mod genres;
 pub mod health;
@@ -24,4 +25,5 @@ pub fn routes() -> Router<AppState> {
         .merge(genres::routes())
         .merge(tags::routes())
         .merge(authors::routes())
+        .merge(collections::routes())
 }