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/Cargo.lock

@@ -541,6 +541,17 @@ version = "0.3.32"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cecba35d7ad927e23624b22ad55235f2239cfa44fd10428eecbeba6d6a717718"
 
+[[package]]
+name = "futures-macro"
+version = "0.3.32"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e835b70203e41293343137df5c0664546da5745f82ec9b84d40be8336958447b"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "futures-sink"
 version = "0.3.32"
@@ -561,6 +572,7 @@ checksum = "389ca41296e6190b48053de0321d02a77f32f8a5d2461dd38762c0593805c6d6"
 dependencies = [
  "futures-core",
  "futures-io",
+ "futures-macro",
  "futures-sink",
  "futures-task",
  "memchr",
@@ -1021,7 +1033,7 @@ checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
 [[package]]
 name = "mangalord"
-version = "0.5.0"
+version = "0.6.0"
 dependencies = [
  "anyhow",
  "argon2",
@@ -1029,8 +1041,11 @@ dependencies = [
  "axum",
  "axum-extra",
  "base64",
+ "bytes",
  "chrono",
  "dotenvy",
+ "futures-core",
+ "futures-util",
  "http-body-util",
  "infer",
  "mime",
@@ -1044,6 +1059,7 @@ dependencies = [
  "thiserror 1.0.69",
  "time",
  "tokio",
+ "tokio-util",
  "tower",
  "tower-http",
  "tracing",
@@ -2028,6 +2044,19 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tokio-util"
+version = "0.7.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9ae9cec805b01e8fc3fd2fe289f89149a9b66dd16786abd8b19cfa7b48cb0098"
+dependencies = [
+ "bytes",
+ "futures-core",
+ "futures-sink",
+ "pin-project-lite",
+ "tokio",
+]
+
 [[package]]
 name = "tower"
 version = "0.5.3"