feat(filter): label endpoint names id values for client-held trees

The SSR embed (65f257f3) covers trees the server renders; one path
still fell through to the flawed fallback — applying a SAVED filter
from a cold bookshelf. That page was rendered before any filter
existed (nothing embedded), and the fallback reused the suggest facet,
whose rows are MEDIA-CONSTRAINED: "feed in [A,B,C] AND title contains
Chapter" labeled only the feeds with a matching episode (the exact
live repro from the original bug report).

- GET /api/filter/labels?field=X&ids=a,b,c resolves names straight
  from the entity tables via the shared ResolveFieldLabels core
  (extracted from FilterTreeLabels): no media join, no tree
  constraint. RLS/visibility ride the request identity — a
  zero-episode subscribed feed resolves its title (pinned; the facet
  path structurally could not), an unsubscribed feed id stays unnamed
  (pinned), non-id fields are 400 (pinned), ids cap at 100.
- Client: resolveIdLabels/resolveLabelsAsync use ONLY the endpoint;
  the quiet suggest-fetch mode is deleted (its only caller was the
  flawed fallback). The saved-filter apply flow resolves through the
  endpoint and rebuilds the chips bar when labels land — chips render
  raw ids only while the round trip is in flight, never stuck.
- 188 web tests green; Go endpoint pins + filter/episode slices green;
  contract_pins documents the endpoint and the fallback removal.

internal/services/filter_labels.go

@@ -29,9 +29,25 @@ import (
 // a legal pick, the resolver only names it.
 // ════════════════════════════════════════════════════════════════════
 
-// maxResolveIDs bounds one field's resolution query (picks accumulate
-// per rule; menus cap far below this).
-const maxResolveIDs = 100
+// MaxResolveIDs bounds one field's resolution query (picks accumulate
+// per rule; menus cap far below this). The label endpoint truncates to
+// the same cap.
+const MaxResolveIDs = 100
+
+// IsIDValuedField — whether the field's values are entity ids (the
+// label endpoint's accepted universe: id-kind + a known source).
+func IsIDValuedField(field string) bool {
+	f, ok := FilterFieldByKey(field)
+	if !ok {
+		return false
+	}
+	return valueKindFor(f.Kind, field) == VKId && f.Suggest != ""
+}
+
+// ParseUUIDValues splits comma-joined picks into uuids, skipping junk.
+func ParseUUIDValues(values []string) []uuid.UUID {
+	return parseUUIDValues(values)
+}
 
 // FilterTreeLabels resolves every id-kind rule value in the tree to its
 // display name. Best-effort by design: a resolve failure degrades to
@@ -64,7 +80,7 @@ func (s *SearchService) FilterTreeLabels(ctx context.Context, userID pgtype.UUID
 				continue
 			}
 			for _, id := range parseUUIDValues(c.Values) {
-				if len(byField[c.Field]) >= maxResolveIDs {
+				if len(byField[c.Field]) >= MaxResolveIDs {
 					return
 				}
 				byField[c.Field] = append(byField[c.Field], id)
@@ -74,48 +90,62 @@ func (s *SearchService) FilterTreeLabels(ctx context.Context, userID pgtype.UUID
 	walk(tree.Children)
 
 	for field, ids := range byField {
-		f, _ := FilterFieldByKey(field)
-		idArgs := make([]uuid.UUID, len(ids))
-		copy(idArgs, ids)
-		var sqlStr string
-		var args []any
-		switch f.Suggest {
-		case suggestLibrary:
-			// The facet's visibility shape, minus the media join: a
-			// hidden library is invisible even by id (family lens).
-			sqlStr = `SELECT l.id::text, l.name
-FROM libraries l
-LEFT JOIN library_visibility lv ON l.id = lv.library_id AND lv.user_id = $2
-WHERE COALESCE(lv.is_visible, true) = true AND l.id = ANY($1)`
-			args = []any{idArgs, userID}
-		case suggestFeed:
-			// RLS-governed (subscription + visibility predicate): an
-			// unsubscribed feed id resolves to nothing.
-			sqlStr = `SELECT id::text, title FROM podcast_feeds WHERE id = ANY($1)`
-			args = []any{idArgs}
-		case suggestCollection:
-			// RLS-governed (app_can_read_collection).
-			sqlStr = `SELECT id::text, name FROM collections WHERE id = ANY($1)`
-			args = []any{idArgs}
-		case suggestAddedBy:
-			sqlStr = `SELECT id::text, username FROM users WHERE id = ANY($1)`
-			args = []any{idArgs}
-		default:
-			continue
+		for k, v := range s.ResolveFieldLabels(ctx, userID, field, ids) {
+			labels[k] = v
 		}
-		rows, err := s.raw.Query(ctx, sqlStr, args...)
-		if err != nil {
-			log.Printf("[filter-labels] resolve %q failed: %v", field, err)
-			continue
+	}
+	return labels
+}
+
+// ResolveFieldLabels names one field's picked ids against its source
+// table — the shared core of the SSR embed and the label endpoint.
+// Unknown fields resolve to nothing (the caller validates).
+func (s *SearchService) ResolveFieldLabels(ctx context.Context, userID pgtype.UUID, field string, ids []uuid.UUID) map[string]string {
+	labels := map[string]string{}
+	f, ok := FilterFieldByKey(field)
+	if !ok {
+		return labels
+	}
+	idArgs := make([]uuid.UUID, len(ids))
+	copy(idArgs, ids)
+	var sqlStr string
+	var args []any
+	switch f.Suggest {
+	case suggestLibrary:
+		// The facet's visibility shape, minus the media join: a
+		// hidden library is invisible even by id (family lens).
+		sqlStr = `SELECT l.id::text, l.name
+FROM libraries l
+LEFT JOIN library_visibility lv ON l.id = lv.library_id AND lv.user_id = $2
+WHERE COALESCE(lv.is_visible, true) = true AND l.id = ANY($1)`
+		args = []any{idArgs, userID}
+	case suggestFeed:
+		// RLS-governed (subscription + visibility predicate): an
+		// unsubscribed feed id resolves to nothing.
+		sqlStr = `SELECT id::text, title FROM podcast_feeds WHERE id = ANY($1)`
+		args = []any{idArgs}
+	case suggestCollection:
+		// RLS-governed (app_can_read_collection).
+		sqlStr = `SELECT id::text, name FROM collections WHERE id = ANY($1)`
+		args = []any{idArgs}
+	case suggestAddedBy:
+		sqlStr = `SELECT id::text, username FROM users WHERE id = ANY($1)`
+		args = []any{idArgs}
+	default:
+		return labels
+	}
+	rows, err := s.raw.Query(ctx, sqlStr, args...)
+	if err != nil {
+		log.Printf("[filter-labels] resolve %q failed: %v", field, err)
+		return labels
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var v, display string
+		if err := rows.Scan(&v, &display); err != nil {
+			break
 		}
-		for rows.Next() {
-			var v, display string
-			if err := rows.Scan(&v, &display); err != nil {
-				break
-			}
-			labels[v] = display
-		}
-		rows.Close()
+		labels[v] = display
 	}
 	return labels
 }