feat: crawler scaffold with chromium launcher (0.22.0)

- crawler module (browser, source trait, jobs, diff) + binary
- chromiumoxide launcher with fetcher feature (auto-downloads
  Chromium on first run, caches under ~/.cache/mangalord/chromium)
- LaunchOptions struct with extra_args, parseable from
  CRAWLER_BROWSER_MODE and CRAWLER_BROWSER_ARGS
- migration 0012 introduces sources, manga_sources,
  chapter_sources, crawler_jobs
- integration tests for headed + headless launch, ipify load+parse,
  and extra-args propagation (all #[ignore], opt-in)

backend/src/crawler/jobs.rs

@@ -0,0 +1,55 @@
+//! Persistent job queue and the four job kinds.
+//!
+//! Backed by Postgres (the `crawler_jobs` table). Workers lease rows
+//! with `SELECT ... FOR UPDATE SKIP LOCKED`, heartbeat via
+//! `leased_until`, and ack by transitioning to `done` (or backoff /
+//! `dead`). Handlers are idempotent so a crash mid-run is recoverable
+//! by replay.
+//!
+//! Scaffold only — the actual queue wrapper and handler dispatch land
+//! once we have the first `Source` impl exercising the pipeline.
+
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+use super::source::DiscoverMode;
+
+#[derive(Clone, Debug, Serialize, Deserialize)]
+#[serde(tag = "kind", rename_all = "snake_case")]
+pub enum JobPayload {
+    /// Walk the source index and enqueue `SyncManga` jobs.
+    Discover {
+        source_id: String,
+        mode: DiscoverMode,
+    },
+    /// Fetch one manga's detail page, upsert metadata, enqueue
+    /// `SyncChapterList`.
+    SyncManga {
+        source_id: String,
+        source_manga_key: String,
+    },
+    /// Diff the chapter list, enqueue `SyncChapterContent` for new
+    /// chapters, soft-drop vanished ones.
+    SyncChapterList {
+        source_id: String,
+        manga_id: Uuid,
+        source_manga_key: String,
+    },
+    /// Download a single chapter's page images into storage.
+    SyncChapterContent {
+        source_id: String,
+        chapter_id: Uuid,
+        source_chapter_key: String,
+    },
+}
+
+#[derive(Clone, Copy, Debug, sqlx::Type, Serialize, Deserialize)]
+#[sqlx(type_name = "text", rename_all = "snake_case")]
+#[serde(rename_all = "snake_case")]
+pub enum JobState {
+    Pending,
+    Running,
+    Done,
+    Failed,
+    Dead,
+}