bugfix: wrap manga + chapter uploads in a DB transaction

Previously a storage failure mid-chapter-upload left a partial chapter
row pointing at a `page_count` that didn't match what was on disk, plus
any successfully-inserted page rows. Same shape for a manga create
where the cover put or cover_image_path UPDATE failed after the manga
row was already inserted.

Fix at the DB layer: open `pool.begin()` at the start of the create,
do all DB writes against `&mut *tx`, commit only after the full
sequence succeeds. If anything before commit fails, the transaction is
rolled back on drop and the DB stays consistent. Bytes already written
to storage on a rolled-back transaction become orphans on disk; a
future reaper can sweep them, and we prioritise DB consistency over
storage tidiness in this branch.

- repo::manga::create / set_cover_image_path: signature changed to
  `impl PgExecutor<'_>` so handlers can pass either `&PgPool` or
  `&mut *tx`. set_cover_image_path is new — replaces the inline
  `UPDATE` in the manga upload handler so the call site stays
  consistent.
- repo::chapter::create / set_page_count: same shape.
- repo::page::create: same.
- api::mangas::create and api::chapters::create both open a
  transaction around their DB writes; storage puts happen inside the
  transaction window (since they must precede the page-row insert), so
  a failed put aborts before commit.

New integration test (api_uploads::chapter_upload_rolls_back_when_
storage_fails_mid_loop) uses a `FailingStorage` helper that errors on
the N-th `put`. With N=1 (page 2 fails), the handler returns 500 and
the chapter + page tables stay empty.

`harness_with_failing_storage` is exposed alongside the existing
`harness` so future tests can reuse it for other fault-injection
cases.

Lockstep version bump to 0.9.3.

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

backend/src/repo/page.rs

@@ -1,13 +1,13 @@
 //! Per-page persistence. Mirrors the rows that `pages` holds.
 
-use sqlx::PgPool;
+use sqlx::{PgExecutor, PgPool};
 use uuid::Uuid;
 
 use crate::domain::Page;
 use crate::error::AppResult;
 
-pub async fn create(
-    pool: &PgPool,
+pub async fn create<'e, E: PgExecutor<'e>>(
+    executor: E,
     chapter_id: Uuid,
     page_number: i32,
     storage_key: &str,
@@ -24,7 +24,7 @@ pub async fn create(
     .bind(page_number)
     .bind(storage_key)
     .bind(content_type)
-    .fetch_one(pool)
+    .fetch_one(executor)
     .await?;
     Ok(row)
 }