feat(store): track when the server last checked a series

Adds latest_checked_at plus DueForLatestCheck and MarkLatestChecked.
The column is kept out of bookmarkColumns on purpose: PUT /bookmarks
decodes a whole Bookmark and Upsert writes every column it knows, so a
client PUT would zero the field and defeat the cooldown.

backend/store.go

@@ -59,6 +59,7 @@ CREATE TABLE IF NOT EXISTS bookmarks (
   favorite           INTEGER NOT NULL DEFAULT 0,
   latest_chapter     TEXT NOT NULL DEFAULT '',
   latest_chapter_num REAL,
+  latest_checked_at  INTEGER NOT NULL DEFAULT 0,
   updated_at         INTEGER NOT NULL
 );`
 
@@ -68,6 +69,10 @@ var addedColumns = []struct{ name, ddl string }{
 	{"favorite", `ALTER TABLE bookmarks ADD COLUMN favorite INTEGER NOT NULL DEFAULT 0`},
 	{"latest_chapter", `ALTER TABLE bookmarks ADD COLUMN latest_chapter TEXT NOT NULL DEFAULT ''`},
 	{"latest_chapter_num", `ALTER TABLE bookmarks ADD COLUMN latest_chapter_num REAL`},
+	// When the server last looked at this series, unix ms; 0 means never, and
+	// sorts first so a new bookmark is picked up on the next tick with no
+	// special case. Deliberately NOT in bookmarkColumns — see MarkLatestChecked.
+	{"latest_checked_at", `ALTER TABLE bookmarks ADD COLUMN latest_checked_at INTEGER NOT NULL DEFAULT 0`},
 }
 
 const bookmarkColumns = `key, site, series_id, title, series_url, cover,
@@ -277,3 +282,54 @@ func (s *Store) Delete(key string) error {
 	}
 	return nil
 }
+
+// DueForLatestCheck returns bookmarks whose server-side latest-chapter check has
+// aged past cutoffMs, least-recently-checked first, at most limit of them.
+//
+// Oldest-first is what keeps the poller fair when the backlog outgrows its
+// throughput: the most neglected series is always next, so a large collection
+// refreshes uniformly slower rather than leaving a tail that never refreshes at
+// all. The userscript sorts its own queue the same way (L453).
+//
+// Bookmarks with no series_url are skipped — there is nothing to fetch, which
+// is the same filter the userscript applies at L452.
+func (s *Store) DueForLatestCheck(cutoffMs int64, limit int) ([]Bookmark, error) {
+	rows, err := s.db.Query(`SELECT `+bookmarkColumns+`
+		FROM bookmarks
+		WHERE series_url IS NOT NULL AND series_url <> ''
+		  AND latest_checked_at <= ?
+		ORDER BY latest_checked_at ASC
+		LIMIT ?`, cutoffMs, limit)
+	if err != nil {
+		return nil, fmt.Errorf("query due bookmarks: %w", err)
+	}
+	defer rows.Close()
+
+	out := []Bookmark{}
+	for rows.Next() {
+		b, err := scanBookmark(rows.Scan)
+		if err != nil {
+			return nil, fmt.Errorf("scan due bookmark: %w", err)
+		}
+		out = append(out, b)
+	}
+	return out, rows.Err()
+}
+
+// MarkLatestChecked records that the server looked at key at ts, whatever the
+// look turned up. Marking a missing key is not an error: the row may have been
+// deleted while a fetch was in flight.
+//
+// This is the one write that does not go through Upsert, and the column is kept
+// out of bookmarkColumns on purpose. PUT /bookmarks/{key} decodes a whole
+// Bookmark from the client and Upsert writes every column it knows about, so a
+// userscript PUT — which has no idea this field exists — would write a zero and
+// reset the cooldown, making the poller re-fetch that series every tick for as
+// long as the user kept reading it.
+func (s *Store) MarkLatestChecked(key string, ts int64) error {
+	if _, err := s.db.Exec(
+		`UPDATE bookmarks SET latest_checked_at = ? WHERE key = ?`, ts, key); err != nil {
+		return fmt.Errorf("mark checked %q: %w", key, err)
+	}
+	return nil
+}