feat(api): admin manga/chapter overview with derived sync state (0.39.0)

Adds GET /api/v1/admin/mangas and /admin/mangas/:id/chapters guarded by
RequireAdmin. Sync state is computed at query time from the existing
crawler signals (manga_sources / chapter_sources / crawler_jobs) — no
new state column is persisted, so the crawler stays the single writer
of these signals.

Per-manga priority: InProgress (in-flight sync_manga or
sync_chapter_list job) > Dropped (all source rows soft-dropped) >
Synced (default; covers user-uploaded mangas with zero source rows).

Per-chapter priority: Downloading (in-flight sync_chapter_content) >
Dropped (all source rows soft-dropped) > Failed (most-recent terminal
job is dead) > NotDownloaded (page_count = 0) > Synced. The Failed
check sits ABOVE NotDownloaded so the more informative "we tried and
it died" state wins over "we never got around to it" — see the
priority comment in repo/admin_view.rs.

Migration 0020 adds a partial index on
crawler_jobs((payload->>'source_manga_key')) for the one job kind
(sync_manga) whose payload doesn't carry manga_id directly — without it
the in-flight detection for a manga falls back to a seqscan over the
job table.

backend/src/api/admin/mangas.rs

@@ -0,0 +1,82 @@
+//! Admin manga/chapter overview with derived sync state.
+//!
+//! Sync state comes from `repo::admin_view`, which joins the manga /
+//! chapter tables with the crawler signals at query time — there is no
+//! persisted sync_state column. See [`repo::admin_view`] for the
+//! derivation priority order.
+
+use axum::extract::{Path, Query, State};
+use axum::routing::get;
+use axum::{Json, Router};
+use serde::Deserialize;
+use uuid::Uuid;
+
+use crate::api::pagination::PagedResponse;
+use crate::app::AppState;
+use crate::auth::extractor::RequireAdmin;
+use crate::domain::MangaSyncState;
+use crate::error::{AppError, AppResult};
+use crate::repo;
+use crate::repo::admin_view::{AdminChapterRow, AdminMangaRow};
+
+pub fn routes() -> Router<AppState> {
+    Router::new()
+        .route("/admin/mangas", get(list_mangas))
+        .route("/admin/mangas/:id/chapters", get(list_chapters))
+}
+
+#[derive(Debug, Deserialize, Default)]
+pub struct ListMangasParams {
+    #[serde(default)]
+    pub search: Option<String>,
+    /// `in_progress` | `dropped` | `synced`. Unrecognised values are a 400.
+    #[serde(default)]
+    pub sync_state: Option<String>,
+    #[serde(default = "default_limit")]
+    pub limit: i64,
+    #[serde(default)]
+    pub offset: i64,
+}
+
+fn default_limit() -> i64 {
+    50
+}
+
+async fn list_mangas(
+    State(state): State<AppState>,
+    _admin: RequireAdmin,
+    Query(params): Query<ListMangasParams>,
+) -> AppResult<Json<PagedResponse<AdminMangaRow>>> {
+    let limit = params.limit.clamp(1, 200);
+    let offset = params.offset.max(0);
+
+    let sync_state = match params.sync_state.as_deref() {
+        None | Some("") => None,
+        Some("in_progress") => Some(MangaSyncState::InProgress),
+        Some("dropped") => Some(MangaSyncState::Dropped),
+        Some("synced") => Some(MangaSyncState::Synced),
+        Some(other) => {
+            return Err(AppError::InvalidInput(format!(
+                "sync_state must be one of in_progress|dropped|synced (got {other:?})"
+            )));
+        }
+    };
+
+    let q = repo::admin_view::ListAdminMangasQuery {
+        search: params.search.filter(|s| !s.trim().is_empty()),
+        sync_state,
+        limit,
+        offset,
+    };
+    let (items, total) = repo::admin_view::list_mangas_with_sync_state(&state.db, &q).await?;
+    Ok(Json(PagedResponse::with_total(items, limit, offset, total)))
+}
+
+async fn list_chapters(
+    State(state): State<AppState>,
+    _admin: RequireAdmin,
+    Path(manga_id): Path<Uuid>,
+) -> AppResult<Json<Vec<AdminChapterRow>>> {
+    let rows = repo::admin_view::list_chapters_with_sync_state(&state.db, manga_id).await?;
+    Ok(Json(rows))
+}