feat: add notifier Sender interface and digest formatter

internal/notifier/digest.go

@@ -0,0 +1,53 @@
+package notifier
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+
+	"naviwatcher/internal/scanner"
+)
+
+// FormatDigest renders newly-found missing releases into a human-readable
+// Telegram message grouped by artist, with per-artist counts and a link to
+// the Web UI dashboard. It is deterministic: artists are sorted by name and
+// releases within an artist are sorted by title.
+func FormatDigest(missing []scanner.MissingRelease, uiBaseURL string) string {
+	if len(missing) == 0 {
+		return "NaviWatcher: no new missing releases found."
+	}
+
+	type entry struct {
+		title string
+	}
+	byArtist := make(map[string][]entry)
+	order := make([]string, 0)
+	for _, r := range missing {
+		if _, ok := byArtist[r.ArtistID]; !ok {
+			order = append(order, r.ArtistID)
+		}
+		byArtist[r.ArtistID] = append(byArtist[r.ArtistID], entry{title: r.Title})
+	}
+	// Stable ordering by ArtistID.
+	sort.Strings(order)
+
+	var b strings.Builder
+	fmt.Fprintf(&b, "NaviWatcher: %d new missing release(s) found:\n\n", len(missing))
+	for _, artistID := range order {
+		entries := byArtist[artistID]
+		titles := make([]string, 0, len(entries))
+		for _, e := range entries {
+			titles = append(titles, e.title)
+		}
+		sort.Strings(titles)
+		fmt.Fprintf(&b, "%s (%d):\n", artistID, len(titles))
+		for _, t := range titles {
+			fmt.Fprintf(&b, "  - %s\n", t)
+		}
+		b.WriteString("\n")
+	}
+	if uiBaseURL != "" {
+		fmt.Fprintf(&b, "View details: %s\n", strings.TrimRight(uiBaseURL, "/"))
+	}
+	return strings.TrimRight(b.String(), "\n")
+}