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/bin/crawler.rs

@@ -0,0 +1,29 @@
+//! Crawler binary.
+//!
+//! Today: a thin shell that launches Chromium via the shared
+//! `crawler::browser` module and exits. Useful as an ad-hoc smoke test
+//! for the launcher in addition to the integration test in
+//! `tests/crawler_browser_smoke.rs`.
+//!
+//! Future: reads config, picks `Source` impls, runs the job loop.
+
+use mangalord::crawler::browser::{self, LaunchOptions};
+use tracing_subscriber::EnvFilter;
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    dotenvy::dotenv().ok();
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| "info,mangalord=debug".into()),
+        )
+        .init();
+
+    let options = LaunchOptions::from_env();
+    tracing::info!(?options, "launching browser");
+    let handle = browser::launch(options).await?;
+    tracing::info!("browser launched; closing");
+    handle.close().await?;
+    Ok(())
+}