feat: notifications say what changed and alert once: a grouped notice names the tracks, grows without re-alerting, and a phone pile lists its titles (M542 #6146-#6151)

- An update to a grouped notice keeps its arrival time and email state, so the
  phone and the email no longer go off again each time it grows.
- Notices carry the names of what changed (newest ten), shown as three and
  "and N more": tracks still missing after the scan's move/replacement fold,
  duplicate groups new to a sweep, songs merged automatically, and the track
  and listener a playback error was for.
- Duplicates found counts only the groups new to that sweep; playback errors
  count reports, not the unresolved total.
- A scan failure says what stopped and links to Admin > Library.
- Android: a pile of more than three alerts lists the newest five titles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/notifications/render.go

@@ -3,6 +3,7 @@ package notifications
 import (
 	"encoding/json"
 	"fmt"
+	"strings"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/words"
 )
@@ -37,10 +38,15 @@ type Payload struct {
 	// Actor is the other person involved: who asked, who flagged.
 	Actor  string `json:"actor,omitempty"`
 	Reason string `json:"reason,omitempty"`
-	// Count is the coalesced kinds' running total.
+	// Count is the coalesced kinds' running total since the notice was last
+	// read.
 	Count int64 `json:"count,omitempty"`
-	// Detail is free text: scan_failed's error message, and what the duplicate
-	// resolver merged for duplicates_resolved.
+	// Items name what Count counts, one each, newest first: "Weezer – Buddy
+	// Holly". A coalesced update puts its own in front and keeps ten (M542),
+	// so the body can say which, not only how many.
+	Items []string `json:"items,omitempty"`
+	// Detail is free text: scan_failed's error message. Rows from before
+	// Items also carry duplicates_resolved's summary here.
 	Detail string `json:"detail,omitempty"`
 }
 
@@ -101,37 +107,52 @@ func Render(kind Kind, payload []byte) Rendered {
 		if p.Count > 1 {
 			title = fmt.Sprintf("Library scan failed %d times", p.Count)
 		}
-		body := p.Detail
-		if body == "" {
-			body = "The last scan stopped with an error."
+		body := "The last scan stopped with an error."
+		if p.Detail != "" {
+			body = "The last scan stopped: " + p.Detail
 		}
-		return Rendered{Title: title, Body: body, Link: "/admin"}
+		return Rendered{Title: title, Body: body, Link: "/admin/library"}
 	case KindTracksMissing:
+		body := "Their files are no longer where the library expects them."
+		if names := nameList(p.Items, p.Count); names != "" {
+			body = "No longer on disk: " + names + "."
+		}
 		return Rendered{
 			Title: plural(p.Count, "track went missing", "tracks went missing"),
-			Body:  "Their files are no longer where the library expects them.",
+			Body:  body,
 			Link:  "/collection/missing",
 		}
 	case KindDuplicatesFound:
+		body := "The duplicate sweep found tracks holding the same recording."
+		if names := nameList(p.Items, p.Count); names != "" {
+			body = "Held more than once: " + names + "."
+		}
 		return Rendered{
-			Title: plural(p.Count, "duplicate group to review", "duplicate groups to review"),
-			Body:  "The duplicate sweep found tracks holding the same recording.",
+			Title: plural(p.Count, "new duplicate to review", "new duplicates to review"),
+			Body:  body,
 			Link:  "/collection/duplicates",
 		}
 	case KindDuplicatesResolved:
 		body := "Extra copies of a song were merged into the one the album keeps."
-		if p.Detail != "" {
+		switch names := nameList(p.Items, p.Count); {
+		case names != "":
+			body = "Extra copies merged into the one the album keeps: " + names + "."
+		case p.Detail != "":
 			body = p.Detail
 		}
 		return Rendered{
-			Title: plural(p.Count, "duplicate resolved automatically", "duplicates resolved automatically"),
+			Title: plural(p.Count, "duplicate merged automatically", "duplicates merged automatically"),
 			Body:  body,
 			Link:  "/collection/duplicates?view=resolved",
 		}
 	case KindPlaybackErrors:
+		body := "A track failed to play for someone."
+		if names := nameList(p.Items, p.Count); names != "" {
+			body = "Failed to play: " + names + "."
+		}
 		return Rendered{
 			Title: plural(p.Count, "playback error reported", "playback errors reported"),
-			Body:  "A track failed to play for someone.",
+			Body:  body,
 			Link:  "/collection/reports",
 		}
 	default:
@@ -139,6 +160,26 @@ func Render(kind Kind, payload []byte) Rendered {
 	}
 }
 
+// namesShown is how many of a notice's items its body names.
+const namesShown = 3
+
+// nameList names the first few items, then how many more of the total there
+// are: "A, B, C and 4 more". Empty when there are no items.
+func nameList(items []string, total int64) string {
+	if len(items) == 0 {
+		return ""
+	}
+	shown := items[:min(len(items), namesShown)]
+	more := max(total, int64(len(items))) - int64(len(shown))
+	if more > 0 {
+		return strings.Join(shown, ", ") + " and " + words.Count(more, "more", "more")
+	}
+	if len(shown) == 1 {
+		return shown[0]
+	}
+	return strings.Join(shown[:len(shown)-1], ", ") + " and " + shown[len(shown)-1]
+}
+
 func plural(n int64, one, many string) string {
 	if n < 1 {
 		// A coalesced kind always carries a count; this only guards a