feat(server): scanner + DeleteTrackFile write library_changes

Wires sync.LogChange into the library mutation sites so /api/library/sync
reflects upserts and deletes.

Architectural pivot: LogChange's signature is now (ctx, dbq.DBTX, ...) so
it works with both *pgxpool.Pool and pgx.Tx. The scanner doesn't run
mutations in explicit transactions, so it pool-binds; delete.go matches.
Tx-bound callers (likes/playlists in subsequent commits) keep atomicity.

Also: sync.FormatUUID centralizes the pgtype.UUID → canonical string
conversion that both the scanner and the sync handler need; library_sync.go
now uses it instead of a local copy.

Best-effort logging on scanner failures (Warn, don't fail the scan): a
LogChange error after a successful upsert is rare and self-healing — the
next scan that touches the entity re-emits the change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/library_sync.go

@@ -168,7 +168,7 @@ func (h *handlers) hydrateUpserts(
 		}
 	}
 
-	userIDStr := uuidToString(userID)
+	userIDStr := syncpkg.FormatUUID(userID)
 
 	if rows := ids[syncpkg.EntityLikeTrack]; len(rows) > 0 {
 		out["like_track"] = scopedLikeRows(rows, userIDStr, "track_id")
@@ -188,7 +188,7 @@ func (h *handlers) hydrateUpserts(
 		}
 		for _, p := range playlists {
 			// Filter: only return playlists owned by this user OR public ones
-			if uuidToString(p.UserID) != userIDStr && !p.IsPublic {
+			if syncpkg.FormatUUID(p.UserID) != userIDStr && !p.IsPublic {
 				continue
 			}
 			b, _ := json.Marshal(p)
@@ -249,32 +249,6 @@ func stringsToUUIDs(strs []string) []pgtype.UUID {
 	return out
 }
 
-// uuidToString renders a pgtype.UUID as the canonical 8-4-4-4-12 hex
-// form. Returns "" if the UUID is not valid.
-func uuidToString(u pgtype.UUID) string {
-	if !u.Valid {
-		return ""
-	}
-	b := u.Bytes
-	return formatUUIDBytes(b)
-}
-
-func formatUUIDBytes(b [16]byte) string {
-	const hex = "0123456789abcdef"
-	out := make([]byte, 36)
-	pos := 0
-	for i := 0; i < 16; i++ {
-		if i == 4 || i == 6 || i == 8 || i == 10 {
-			out[pos] = '-'
-			pos++
-		}
-		out[pos] = hex[b[i]>>4]
-		out[pos+1] = hex[b[i]&0x0f]
-		pos += 2
-	}
-	return string(out)
-}
-
 // splitOnce returns [before, after] split at the first occurrence of sep,
 // or [s] if sep doesn't appear. Used for composite-key parsing.
 func splitOnce(s, sep string) []string {