feat: multipart manga + chapter uploads with magic-byte MIME sniff

POST /api/v1/mangas and POST /api/v1/mangas/{id}/chapters now accept
multipart/form-data, gated by CurrentUser:

- /mangas: required `metadata` part (NewManga JSON) + optional `cover`
  image part.
- /mangas/{id}/chapters: required `metadata` (NewChapter JSON) + one or
  more `page` parts ordered by arrival. Returns 404 if the parent manga
  doesn't exist, 409 on duplicate (manga_id, number).

MIME is sniffed via the `infer` crate (magic bytes), not the
client-supplied filename or Content-Type. Whitelist:
jpeg / png / webp / gif / avif. Anything else → 415
unsupported_media_type. The stored key's extension is derived from the
sniffed type so a "page1.png" that's actually a JPEG lands as `.jpg`.

Size cap is two-layer:
- Request body cap (config.max_request_bytes, default 200 MiB) enforced
  by axum's DefaultBodyLimit before the handler sees the request.
- Per-image-part cap (config.max_file_bytes, default 20 MiB) enforced
  after reading the part, so a single oversized image can't pass even
  if the total request fits.

Storage keys follow the layout documented in CLAUDE.md:
- mangas/{manga_id}/cover.{ext}
- mangas/{manga_id}/chapters/{chapter_id}/pages/{nnnn}.{ext} (1-indexed).

AppError grows PayloadTooLarge/UnsupportedMediaType/ValidationFailed
(413 / 415 / 422). ValidationFailed carries a `details` JSON object the
client can use to highlight bad fields (e.g. {"title":"required"}).
Top-level matching in code() stays exhaustive.

Backend coverage in tests/api_uploads.rs (10 cases):
- create_manga_with_cover_stores_image — file is reachable via
  /api/v1/files/{key} with the right Content-Type.
- create_manga_without_cover_leaves_path_null.
- create_manga_rejects_non_image_cover_with_415 — PDF claimed as png.
- create_manga_rejects_oversized_cover_with_413.
- create_chapter_with_pages_stores_each — extension derived from
  sniffed MIME, files reachable in arrival order.
- create_chapter_rejects_when_no_pages_with_422 — details.page set.
- create_chapter_rejects_renamed_non_image_page → 415.
- create_chapter_returns_409_on_duplicate_number.
- create_chapter_requires_authentication → 401.
- create_chapter_under_unknown_manga_is_404.

Existing tests/api_mangas.rs is migrated to multipart; the create
response is now 201 Created. tests/common::MultipartBuilder builds the
body by hand so the test crate stays free of HTTP-client deps.

Frontend lib/api/mangas.ts: createManga now sends FormData (metadata +
optional cover Blob). Browser fills in the boundary header automatically.
Vitest asserts the FormData structure via FileReader (jsdom doesn't
implement Blob.text()).

E2E tests wait for the post-hydration nav-login link before
interacting with the login form, fixing a flake where pre-hydration
clicks would submit via the browser default and bypass our handler.

Lockstep version bump to 0.5.0.

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

backend/src/config.rs

@@ -17,12 +17,33 @@ impl Default for AuthConfig {
     }
 }
 
+#[derive(Clone, Debug)]
+pub struct UploadConfig {
+    /// Total request size cap, enforced by axum's DefaultBodyLimit on the
+    /// upload routes. Rejected requests get a 413.
+    pub max_request_bytes: usize,
+    /// Per-image-part size cap, enforced after the part is read. Lets us
+    /// reject a single oversized cover/page without failing the whole
+    /// request just because the total happens to fit.
+    pub max_file_bytes: usize,
+}
+
+impl Default for UploadConfig {
+    fn default() -> Self {
+        Self {
+            max_request_bytes: 200 * 1024 * 1024, // 200 MiB
+            max_file_bytes: 20 * 1024 * 1024,     // 20 MiB
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct Config {
     pub database_url: String,
     pub bind_address: String,
     pub storage_dir: PathBuf,
     pub auth: AuthConfig,
+    pub upload: UploadConfig,
     pub cors_allowed_origins: Vec<String>,
 }
 
@@ -43,6 +64,10 @@ impl Config {
                     .filter(|s| !s.is_empty()),
                 session_ttl_days: env_i64("SESSION_TTL_DAYS", 30),
             },
+            upload: UploadConfig {
+                max_request_bytes: env_usize("MAX_REQUEST_BYTES", 200 * 1024 * 1024),
+                max_file_bytes: env_usize("MAX_FILE_BYTES", 20 * 1024 * 1024),
+            },
             cors_allowed_origins: std::env::var("CORS_ALLOWED_ORIGINS")
                 .ok()
                 .map(|s| {
@@ -70,3 +95,10 @@ fn env_i64(name: &str, default: i64) -> i64 {
         .and_then(|s| s.parse().ok())
         .unwrap_or(default)
 }
+
+fn env_usize(name: &str, default: usize) -> usize {
+    std::env::var(name)
+        .ok()
+        .and_then(|s| s.parse().ok())
+        .unwrap_or(default)
+}