feat: incremental crawl mode with seed-completion gate (0.33.0)

Daemon now auto-detects mode per source: Backfill until the first
full walk records `seed_completed:<source>` in `crawler_state`, then
Incremental (newest-first, stops after N consecutive Unchanged
upserts). `CRAWLER_MODE` overrides to a fixed mode; CLI rejects
`auto` since it has no pre-run DB state.

`Source::discover` returns a lazy `DiscoverWalk` so Incremental can
break out mid-walk without prefetching pages. The drop pass and seed
marker are now gated on a true full walk — fixes a latent soft-drop
of the index tail under partial sweeps.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/repo/crawler.rs

@@ -412,6 +412,53 @@ pub async fn sync_manga_chapters(
     Ok(diff)
 }
 
+/// Record that a complete Backfill walk has finished for `source_id`.
+/// The presence of this row is what the daemon's mode auto-detection
+/// uses to flip from Backfill to Incremental on subsequent ticks.
+///
+/// Keyed `seed_completed:<source_id>` in `crawler_state`. JSON payload
+/// stores the timestamp so we can surface "last fully reseeded at" in
+/// future ops tooling without another migration.
+pub async fn mark_seed_completed(
+    pool: &PgPool,
+    source_id: &str,
+    at: DateTime<Utc>,
+) -> sqlx::Result<()> {
+    let key = format!("seed_completed:{source_id}");
+    sqlx::query(
+        "INSERT INTO crawler_state (key, value, updated_at) \
+         VALUES ($1, $2, now()) \
+         ON CONFLICT (key) DO UPDATE \
+            SET value = EXCLUDED.value, updated_at = now()",
+    )
+    .bind(&key)
+    .bind(serde_json::json!({ "at": at.to_rfc3339() }))
+    .execute(pool)
+    .await?;
+    Ok(())
+}
+
+/// Read the timestamp written by [`mark_seed_completed`], if any.
+/// `None` means no complete Backfill has ever finished for this
+/// source — the daemon should run Backfill on the next tick.
+pub async fn seed_completed_at(
+    pool: &PgPool,
+    source_id: &str,
+) -> sqlx::Result<Option<DateTime<Utc>>> {
+    let key = format!("seed_completed:{source_id}");
+    let row: Option<serde_json::Value> =
+        sqlx::query_scalar("SELECT value FROM crawler_state WHERE key = $1")
+            .bind(&key)
+            .fetch_optional(pool)
+            .await?;
+    Ok(row.and_then(|v| {
+        v.get("at")
+            .and_then(|s| s.as_str())
+            .and_then(|s| DateTime::parse_from_rfc3339(s).ok())
+            .map(|dt| dt.with_timezone(&Utc))
+    }))
+}
+
 pub async fn mark_dropped_mangas(
     pool: &PgPool,
     source_id: &str,