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/api/mangas.rs

@@ -94,19 +94,24 @@ async fn create(
     })?;
     validate_new_manga(&metadata)?;
 
-    let mut manga = repo::manga::create(&state.db, metadata).await?;
+    // Transactional create. If the cover put or the cover_image_path
+    // UPDATE fails, the manga row is rolled back so a half-uploaded
+    // cover doesn't leave a manga without one stuck pointing at it.
+    // Bytes already written to storage on a rolled-back transaction
+    // become orphans on disk — a future reaper can sweep them; we
+    // prioritise DB consistency over storage tidiness here.
+    let mut tx = state.db.begin().await?;
+    let mut manga = repo::manga::create(&mut *tx, metadata).await?;
 
     if let Some(img) = cover {
         let key = format!("mangas/{}/cover.{}", manga.id, img.ext);
         state.storage.put(&key, &img.bytes).await?;
-        sqlx::query("UPDATE mangas SET cover_image_path = $1, updated_at = now() WHERE id = $2")
-            .bind(&key)
-            .bind(manga.id)
-            .execute(&state.db)
-            .await?;
+        repo::manga::set_cover_image_path(&mut *tx, manga.id, &key).await?;
         manga.cover_image_path = Some(key);
     }
 
+    tx.commit().await?;
+
     Ok((StatusCode::CREATED, Json(manga)))
 }