Make semantic search and reranking optional, use site meta descriptions for snippets

- Add semantic_search setting to toggle AI-powered search on/off
- Skip embedding generation, hybrid search, and model preloading when disabled
- Use site owner's meta description as snippet instead of heuristic extraction
- Remove _generate_summary() and snippet() - no more generated snippets
- Show reranker/reindex controls grayed out when semantic search is off
- AI dependencies (onnxruntime, hnswlib, etc.) are now fully optional

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -74,6 +74,9 @@ def ensure_rns_config(config_dir):
 
 def _preload_embeddings():
     """Pre-load the embedding model and build the HNSW index in background."""
+    if get_setting("semantic_search", "1") != "1":
+        print("Semantic search disabled.")
+        return
     try:
         from embeddings import _get_session, _get_reranker, build_index
         _get_session()  # downloads model on first run, loads ONNX session