feat: streaming files endpoint + reader pages + chapter pages metadata

Backend:
- Migration 0003_pages.sql adds a `pages` table (id, chapter_id,
  page_number, storage_key, content_type) with a unique (chapter_id,
  page_number). New table because chapter pages can have different MIME
  types per page; reconstructing keys from a single template would
  break the moment a chapter mixes png and jpg pages.
- `domain::Page` + `repo::page` (create + list_for_chapter).
- The chapter upload handler now inserts one page row per part as it
  writes the bytes to storage.
- GET /api/v1/mangas/{id}/chapters/{n}/pages returns `{pages: [...]}`
  with the storage_key clients need to construct image URLs. 404 if
  the manga or chapter doesn't exist; reads are public.

Storage trait grows `get_stream(&str) -> StreamingFile` returning a
`Pin<Box<dyn Stream<Item = io::Result<Bytes>> + Send>>` + size. The
local backend implements via `tokio::fs::File` + `tokio_util::io::
ReaderStream` with a 64 KiB chunk size. GET /api/v1/files/*key now
streams via `axum::body::Body::from_stream` instead of buffering — the
test asserts a 200 KiB file emits >1 frame end-to-end through the
router.

Frontend:
- lib/api/client.ts gains `fileUrl(key)` so components don't
  reconstruct the `/api/v1/files/...` path manually.
- lib/api/chapters.ts gains `ChapterPage` type + `getChapterPages` (the
  type is named ChapterPage to avoid colliding with `Page` from
  client.ts, which is the pagination envelope).
- /manga/[id]/+page.svelte: overview with cover, title, author,
  description, chapter list, and a disabled bookmark control (real
  bookmarking lands in feat/bookmarks). Responsive at 640 px.
- /manga/[id]/chapter/[n]/+page.svelte: paginated reader. Current page
  loads eagerly; next page is preloaded in a hidden img so navigation
  feels instant. Keyboard handler maps ArrowRight/j/Space → next,
  ArrowLeft/k → prev, Home/End → first/last; skips when the user is
  typing in an input. Focus ring on the prev/next buttons.
- SSR is disabled on both routes via `export const ssr = false` so the
  client-only fetch flow doesn't need to be replicated server-side; the
  routes are interactive features, not SEO surfaces.
- E2E (e2e/reader.spec.ts): overview shows the title/cover/chapter
  list; reader pages through three pages via ArrowRight, j, k, and
  ArrowLeft, and the preload img holds the page-2 src on initial load.

Lockstep version bump to 0.6.0.

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

backend/src/domain/mod.rs

@@ -2,6 +2,7 @@ pub mod api_token;
 pub mod bookmark;
 pub mod chapter;
 pub mod manga;
+pub mod page;
 pub mod session;
 pub mod user;
 
@@ -9,5 +10,6 @@ pub use api_token::ApiToken;
 pub use bookmark::Bookmark;
 pub use chapter::Chapter;
 pub use manga::Manga;
+pub use page::Page;
 pub use session::Session;
 pub use user::User;

backend/src/domain/page.rs

@@ -0,0 +1,12 @@
+use serde::Serialize;
+use sqlx::FromRow;
+use uuid::Uuid;
+
+#[derive(Debug, Clone, Serialize, FromRow)]
+pub struct Page {
+    pub id: Uuid,
+    pub chapter_id: Uuid,
+    pub page_number: i32,
+    pub storage_key: String,
+    pub content_type: String,
+}