fix: a fresh app install syncs the whole library, not the last 30 days of changes (#6778)

The change feed dropped every row older than 30 days, and an app syncing
from cursor 0 builds its library from the feed, so a fresh install, cleared
data or a 410 reset got only what had changed lately.

The compactor now keeps each entity's latest change however old. It drops a
row once a newer one for the same entity exists, and a delete once it is 30
days old, raising a stored mark (library_changes_trimmed) past it; an app
whose cursor is below the mark is sent back to 0, as before. Migration 0101
adds the index the compactor needs, seeds the mark where the old compactor
left off, and gives every artist, album and track whose latest row is not an
upsert a fresh one, so the rows the old compactor dropped come back. The
backup carries the mark with the feed.

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

internal/api/library_sync.go

@@ -36,7 +36,9 @@ type syncResponse struct {
 // Responses:
 //   - 204 No Content — no changes since cursor
 //   - 200 OK         — JSON syncResponse with upserts, deletes, new cursor
-//   - 410 Gone       — cursor older than oldest log row; client must reset
+//   - 410 Gone       — the client must reset its cursor to 0: the cursor is
+//     older than a delete the log has dropped, or past anything this server
+//     has handed out
 //   - 401 / 500      — standard envelope errors
 func (h *handlers) handleLibrarySync(w http.ResponseWriter, r *http.Request) {
 	user, ok := requireUser(w, r)
@@ -52,16 +54,17 @@ func (h *handlers) handleLibrarySync(w http.ResponseWriter, r *http.Request) {
 
 	q := dbq.New(h.pool)
 
-	// 410 path: if the requested cursor is older than the oldest row,
-	// tell the client to reset. (No compaction job in this slice; the
-	// path exists for future compaction support.)
+	// The log keeps every entity's latest change, so a client syncing from
+	// 0 gets the whole library. It drops deletes older than its retention,
+	// and a cursor below the newest dropped one may have missed it: the
+	// client resets and syncs again from 0.
 	if since > 0 {
-		minCursor, err := q.GetMinLibraryChangeCursor(ctx)
+		trimmed, err := q.GetLibraryChangesTrimmedThrough(ctx)
 		if err != nil {
-			writeErrWithLog(w, h.logger, "library_sync: GetMinLibraryChangeCursor", err)
+			writeErrWithLog(w, h.logger, "library_sync: GetLibraryChangesTrimmedThrough", err)
 			return
 		}
-		if minCursor > 0 && since+1 < minCursor {
+		if since < trimmed {
 			writeErr(w, &apierror.Error{
 				Status:  http.StatusGone,
 				Code:    "cursor_too_old",