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/router/search.go

@@ -16,6 +16,10 @@ func registerSearchRoutes(cfg *Config) {
 	// JWT middleware for protected routes
 	jwtMiddleware := createJWTMiddleware(cfg)
 	protected := e.Group("/api", jwtMiddleware)
+	// Filter label resolution (2026-11-01): names id-kind filter values
+	// from their source tables for trees the server didn't render (the
+	// cold saved-filter apply). Visibility/RLS ride the request identity.
+	protected.GET("/filter/labels", cfg.MediaHandler.ResolveFilterLabels)
 	// Search endpoint - returns JSON or HTML based on HX-Request header
 	protected.GET("/media-items/search", func(c *echo.Context) error {
 		// Check if HTMX request (expects HTML)