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>

embeddings.py

@@ -507,7 +507,7 @@ def hybrid_search(query_text, bm25_ranked_ids, limit=10, db=None, use_reranker=F
 
 def reindex_all(db=None, progress_callback=None):
     """Re-embed all pages and regenerate all summaries. Rebuilds HNSW index."""
-    from db import get_db, return_db, _generate_summary
+    from db import get_db, return_db
     own_db = db is None
     if own_db:
         db = get_db()
@@ -523,11 +523,6 @@ def reindex_all(db=None, progress_callback=None):
         total = len(rows)
         for i, row in enumerate(rows):
             store_embeddings(row["id"], row["title"], row["body"], db)
-            # Only regenerate summary if missing
-            if not row["summary"]:
-                summary = _generate_summary(row["title"], row["body"])
-                db.execute("UPDATE pages SET summary = ? WHERE id = ?", (summary, row["id"]))
-                db.commit()
             if progress_callback:
                 progress_callback(i + 1, total)