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/mangas.rs

@@ -9,7 +9,8 @@ use uuid::Uuid;
 use crate::api::pagination::PagedResponse;
 use crate::app::AppState;
 use crate::auth::extractor::CurrentUser;
-use crate::domain::manga::{MangaCard, MangaDetail, MangaPatch, NewManga, Patch};
+use crate::domain::manga::{MangaCard, MangaDetail, MangaPatch, NewManga};
+use crate::domain::patch::Patch;
 use crate::domain::tag::TagRef;
 use crate::error::{AppError, AppResult};
 use crate::repo;