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/domain/collection.rs

@@ -0,0 +1,50 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use sqlx::FromRow;
+use uuid::Uuid;
+
+use super::patch::Patch;
+
+#[derive(Debug, Clone, Serialize, Deserialize, FromRow)]
+pub struct Collection {
+    pub id: Uuid,
+    pub user_id: Uuid,
+    pub name: String,
+    pub description: Option<String>,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: DateTime<Utc>,
+}
+
+/// Shape returned by `GET /me/collections`. Enriched with the manga
+/// count and up to three sample cover paths so a collection card can
+/// render without extra round-trips.
+#[derive(Debug, Clone, Serialize, FromRow)]
+pub struct CollectionSummary {
+    pub id: Uuid,
+    pub user_id: Uuid,
+    pub name: String,
+    pub description: Option<String>,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: DateTime<Utc>,
+    pub manga_count: i64,
+    /// Cover image keys of up to three sample mangas (newest-added
+    /// first). `Vec<String>` rather than `Option<...>` so an empty
+    /// collection renders as `[]` rather than `null`.
+    pub sample_covers: Vec<String>,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct NewCollection {
+    pub name: String,
+    #[serde(default)]
+    pub description: Option<String>,
+}
+
+#[derive(Debug, Clone, Deserialize, Default)]
+pub struct CollectionPatch {
+    pub name: Option<String>,
+    /// Three-state: missing key leaves description alone; explicit
+    /// `null` clears it; a string sets it. See `Patch`.
+    #[serde(default)]
+    pub description: Patch<String>,
+}