fix: address code review findings

- Start Web UI before the blocking initial sync so the dashboard is
  reachable during the (rate-limited, potentially multi-minute) first
  sync; fold the immediate sync into startPeriodicSync's overlap guard
  so it can never race a concurrent tick over the shared DB / MB client.
- Make MarkNotificationSent idempotent: INSERT OR IGNORE for same-second
  PK collisions, and explicitly swallow FK violations when a release was
  pruned by a concurrent re-sync. Prevents a single vanished/duplicate
  release from aborting the digest mark-sent loop and re-sending.
- Do not abort NotifyOnce's mark-sent loop on a single failure; log and
  continue so every release in the batch is marked.
- NULL-safe reads: COALESCE(type,''), COALESCE(release_date,'') in the
  external_releases and unnotified readers to match the cache reader.
- Update/extend tests for the new idempotency and startup contracts.

internal/database/notifications.go

@@ -1,16 +1,37 @@
 package database
 
 import (
+	"errors"
 	"fmt"
+
+	sqlite3 "github.com/mattn/go-sqlite3"
 )
 
 // MarkNotificationSent records that a notification has been sent for the given RGID.
+//
+// Uses INSERT OR IGNORE so a pre-existing marker for the same RGID (a
+// same-second re-notify colliding on the (rgid, sent_at) primary key) is a
+// no-op rather than an error: the marker's presence, not its exact timestamp,
+// is what matters for idempotency.
+//
+// A concurrent re-sync that prunes the external_releases row before this insert
+// would violate the FK constraint. OR IGNORE does NOT downgrade FK violations
+// in this SQLite build, so the FK error is caught explicitly and treated as a
+// benign no-op ("the release is already gone"). This ensures a single vanished
+// release cannot abort a whole digest's mark-sent loop and trigger duplicate
+// notifications on the next run.
 func MarkNotificationSent(db *DB, rgid string) error {
 	_, err := db.Conn().Exec(
-		"INSERT INTO notifications_sent (rgid) VALUES (?)",
+		"INSERT OR IGNORE INTO notifications_sent (rgid) VALUES (?)",
 		rgid,
 	)
 	if err != nil {
+		var sqliteErr sqlite3.Error
+		if errors.As(err, &sqliteErr) && sqliteErr.Code == sqlite3.ErrConstraint &&
+			sqliteErr.ExtendedCode == sqlite3.ErrConstraintForeignKey {
+			// Release row was pruned concurrently; nothing to mark.
+			return nil
+		}
 		return fmt.Errorf("mark notification sent: %w", err)
 	}
 	return nil
@@ -33,7 +54,7 @@ func IsNotificationSent(db *DB, rgid string) (bool, error) {
 // artists are excluded so the digest honors the monitoring contract.
 func GetUnnotifiedReleases(db *DB) ([]ExternalRelease, error) {
 	rows, err := db.Conn().Query(`
-		SELECT e.rgid, e.artist_id, e.title, e.type, e.release_date, e.is_ignored
+		SELECT e.rgid, e.artist_id, e.title, COALESCE(e.type,''), COALESCE(e.release_date,''), e.is_ignored
 		FROM external_releases e
 		JOIN artist_settings s ON e.artist_id = s.id
 		LEFT JOIN notifications_sent n ON e.rgid = n.rgid