perf(api,indexer): optimiser pages, thumbnails, watcher et robustesse fd

- Pages: mode Original (zero-transcoding), ETag/304, cache index CBZ,
  préfetch next 2 pages, filtre Triangle par défaut
- Thumbnails: DCT scaling JPEG via jpeg-decoder (decode 7x plus rapide),
  img.thumbnail() pour resize, support format Original, fix JPEG RGBA8
- API fallback thumbnail: OutputFormat::Original + DCT scaling au lieu
  de WebP full-decode, retour (bytes, content_type) dynamique
- Watcher: remplacement notify par poll léger sans inotify/fd,
  skip poll quand job actif, snapshots en mémoire
- Jobs: mutex exclusif corrigé (tous statuts actifs, tous types exclusifs)
- Robustesse: suppression fs::canonicalize (problèmes fd Docker),
  list_folders avec erreurs explicites, has_children default true
- Backoffice: FormRow items-start pour alignement inputs avec helper text,
  labels settings clarifiés

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/indexer/src/job.rs

@@ -37,27 +37,57 @@ pub async fn cleanup_stale_jobs(pool: &PgPool) -> Result<()> {
     Ok(())
 }
 
+/// Job types that modify book/thumbnail data and must not run concurrently.
+const EXCLUSIVE_JOB_TYPES: &[&str] = &[
+    "rebuild",
+    "full_rebuild",
+    "scan",
+    "thumbnail_rebuild",
+    "thumbnail_regenerate",
+];
+
+/// Active statuses (job is still in progress, not just queued).
+const ACTIVE_STATUSES: &[&str] = &[
+    "running",
+    "extracting_pages",
+    "generating_thumbnails",
+];
+
 pub async fn claim_next_job(pool: &PgPool) -> Result<Option<(Uuid, Option<Uuid>)>> {
     let mut tx = pool.begin().await?;
 
+    // Check if any exclusive job is currently active
+    let has_active_exclusive: bool = sqlx::query_scalar(
+        r#"
+        SELECT EXISTS(
+            SELECT 1 FROM index_jobs
+            WHERE status = ANY($1)
+              AND type = ANY($2)
+        )
+        "#,
+    )
+    .bind(ACTIVE_STATUSES)
+    .bind(EXCLUSIVE_JOB_TYPES)
+    .fetch_one(&mut *tx)
+    .await?;
+
     let row = sqlx::query(
         r#"
         SELECT j.id, j.type, j.library_id
         FROM index_jobs j
         WHERE j.status = 'pending'
           AND (
-            (j.type IN ('rebuild', 'full_rebuild') AND NOT EXISTS (
-                SELECT 1 FROM index_jobs
-                WHERE status = 'running'
-                  AND type IN ('rebuild', 'full_rebuild')
-            ))
+            -- Exclusive jobs: only if no other exclusive job is active
+            (j.type = ANY($1) AND NOT $2::bool)
             OR
-            j.type NOT IN ('rebuild', 'full_rebuild')
+            -- Non-exclusive jobs (cbr_to_cbz): can always run
+            j.type != ALL($1)
           )
         ORDER BY
             CASE j.type
                 WHEN 'full_rebuild' THEN 1
                 WHEN 'rebuild' THEN 2
+                WHEN 'scan' THEN 2
                 ELSE 3
             END,
             j.created_at ASC
@@ -65,6 +95,8 @@ pub async fn claim_next_job(pool: &PgPool) -> Result<Option<(Uuid, Option<Uuid>)
         LIMIT 1
         "#,
     )
+    .bind(EXCLUSIVE_JOB_TYPES)
+    .bind(has_active_exclusive)
     .fetch_optional(&mut *tx)
     .await?;
 
@@ -74,30 +106,8 @@ pub async fn claim_next_job(pool: &PgPool) -> Result<Option<(Uuid, Option<Uuid>)
     };
 
     let id: Uuid = row.get("id");
-    let job_type: String = row.get("type");
     let library_id: Option<Uuid> = row.get("library_id");
 
-    if job_type == "rebuild" || job_type == "full_rebuild" {
-        let has_running_rebuild: bool = sqlx::query_scalar(
-            r#"
-            SELECT EXISTS(
-                SELECT 1 FROM index_jobs
-                WHERE status = 'running'
-                  AND type IN ('rebuild', 'full_rebuild')
-                  AND id != $1
-            )
-            "#,
-        )
-        .bind(id)
-        .fetch_one(&mut *tx)
-        .await?;
-
-        if has_running_rebuild {
-            tx.rollback().await?;
-            return Ok(None);
-        }
-    }
-
     sqlx::query(
         "UPDATE index_jobs SET status = 'running', started_at = NOW(), error_opt = NULL WHERE id = $1",
     )