feat: verify acceptance criteria and document sync config

Add a scan->notify->web end-to-end smoke test against a seeded :memory: DB,
fix .gitignore so the cmd/naviwatcher source dir is not excluded by the
root binary rule, and document the new sync.interval field in config.yaml.example.

config.yaml.example

@@ -22,3 +22,10 @@ telegram:
 
 scanner:
   fuzzy_threshold: 0.85
+
+# Periodic sync+scan pipeline: how often NaviWatcher pulls artists/albums from
+# Navidrome, resolves MusicBrainz discographies, and re-runs the scanner.
+# Accepts any duration Go's time.ParseDuration understands (e.g. "6h", "30m").
+# Defaults to 6h when omitted.
+sync:
+  interval: 6h