Fase B6: temporal relevance decay and vector-based "similar pages"

- combine-results now applies a bounded (never below 0.5x) exponential
  decay based on :block/updated-at, so more recently touched blocks/pages
  rank slightly higher for otherwise-equal matches.
- New search/similar-blocks reuses the existing zvec index: fetch a
  block's own already-indexed embedding (new :fetch capability on
  open-vector-index, via zvec's fetchSync) and re-query the index with
  it instead of embedding a fresh query string.
- Exposed as a :similar-pages render-resource (worker/handler/render-resource/similar.cljs),
  consistent with how linked/unlinked references are already wired, plus
  a raw :thread-api/search-similar-blocks for non-UI callers.
- New "Similar pages" section on the page view, above the existing
  linked/unlinked references.

src/main/frontend/worker/handler/render_resource/engine.cljs

@@ -6,6 +6,7 @@
             [frontend.worker.handler.render-resource.common :as common]
             [frontend.worker.handler.render-resource.property :as property]
             [frontend.worker.handler.render-resource.query :as query]
+            [frontend.worker.handler.render-resource.similar :as similar]
             [frontend.worker.handler.render-resource.view :as view]
             [frontend.worker.state :as worker-state]))
 
@@ -14,6 +15,7 @@
          property/resource-renderers
          view/resource-renderers
          query/resource-renderers
+         similar/resource-renderers
          {:block-sync-conflicts
           (common/renderer nil
                            (fn [_db resource-key _runtime]