bugfix: second-pass audit follow-ups (N1-N4)

Four small follow-ups from the second-pass audit:

- N1: `manga_upload_rolls_back_when_cover_storage_fails` covers the
  manga-side of the transactional rollback path. The chapter case had
  a `FailingStorage` regression test already; this completes the
  symmetric pair. With fail-on-put-index=0, the cover put fails on
  the first call, the transaction aborts, and `SELECT count(*) FROM
  mangas WHERE title = 'Berserk'` is 0.

- N2: The SvelteKit proxy now catches network-layer failures from the
  upstream `fetch` (DNS / connection refused / TLS handshake) and
  returns a 502 with the standard error envelope
  (`code: 'upstream_unavailable'`) instead of letting SvelteKit's
  generic 500 HTML page through. `client.ts` can `.json()` the result
  cleanly so callers see a real ApiError with a meaningful code. The
  underlying cause is logged via `console.error` for the operator.
  Test in hooks.server.test.ts asserts the 502, the JSON envelope, and
  that `resolve` is not called (the proxy short-circuits).

- N3: `GET /api/v1/files/*key` now sets
  `X-Content-Type-Options: nosniff`. The upload-time magic-byte sniff
  is authoritative for what we declare as Content-Type; `nosniff`
  makes the contract explicit so older user-agents can't try to
  re-detect HTML/JS in a polyglot file that survived the sniff. Test
  in api_uploads.rs asserts the header.

- N4: The /bookmarks page used `{#if b.page}` to gate the "— page N"
  display, which falsy-elided a legitimate `page == 0`. Backend now
  rejects `page < 1` for new bookmarks (already shipped in 0.9.4),
  but any pre-0.9.4 row with page=0 still rendered without its
  number. Strengthened to `{#if b.page != null && b.page > 0}`.

Lockstep version bump to 0.10.1.

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

backend/Cargo.lock

@@ -1033,7 +1033,7 @@ checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
 [[package]]
 name = "mangalord"
-version = "0.9.4"
+version = "0.10.0"
 dependencies = [
  "anyhow",
  "argon2",