fix(search): defensive _get_bm25 guard, null-safe text_snippet

app/api/search.py

@@ -34,8 +34,11 @@ class SearchResult(BaseModel):
 
 
 def _get_bm25() -> BM25Index:
-    from app.main import _bm25
-    return _bm25
+    import app.main as _main
+    bm25 = getattr(_main, "_bm25", None)
+    if bm25 is None:
+        raise RuntimeError("BM25 index not initialised — app.main not loaded")
+    return bm25
 
 
 def _get_db_path() -> str:
@@ -57,7 +60,7 @@ def search(
             chunk_id=h.chunk_id,
             doc_id=h.doc_id,
             page_number=h.page_number,
-            text_snippet=h.text[:300],
+            text_snippet=(h.text or "")[:300],
             bm25_score=h.score,
         )
         for h in hits