Fix NOT NULL violation on empty splits and increase search results to k=6

Empty documents after sanitization caused aadd_documents to issue a
DEFAULT VALUES insert. Guard with an emptiness check. Also increase
similarity search k from 2 to 6 so multi-word queries like full names
have better recall.

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

blueprints/rag/logic.py

@@ -325,6 +325,7 @@ async def sync_obsidian_documents() -> dict[str, int]:
         if documents:
             splits = text_splitter.split_documents(documents)
             splits = _sanitize_documents(splits)
+            if splits:
                 vector_store = _get_vector_store()
                 await vector_store.aadd_documents(documents=splits)
 
@@ -336,7 +337,7 @@ async def sync_obsidian_documents() -> dict[str, int]:
 
 async def query_vector_store(query: str):
     vector_store = _get_vector_store()
-    retrieved_docs = await vector_store.asimilarity_search(query, k=2)
+    retrieved_docs = await vector_store.asimilarity_search(query, k=6)
     serialized = "\n\n".join(
         (f"Source: {doc.metadata}\nContent: {doc.page_content}")
         for doc in retrieved_docs