fix: cap sqlite-vec k value at 4096 to prevent knn query crash

sqlite-vec enforces a hard limit of 4096 on the k parameter in knn
queries. When a project grows beyond 4096 vector chunks the computed
candidate_limit = max(semantic_vector_k, (limit+offset)*10) can exceed
this, causing OperationalError: k value in knn query too large.

Cap candidate_limit at SQLITE_VEC_MAX_K = 4096 inside
SQLiteSearchRepository._run_vector_query() — the one place in the SQLite
backend where k is passed to sqlite-vec. Adds a regression test that
verifies the cap is enforced for a limit value that would otherwise
produce candidate_limit > 4096.

Fixes #604

Co-authored-by: bm-clawd <bm-clawd@users.noreply.github.com>
Signed-off-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>

src/basic_memory/repository/sqlite_search_repository.py

@@ -28,6 +28,10 @@ from basic_memory.repository.metadata_filters import parse_metadata_filters, bui
 from basic_memory.repository.semantic_errors import SemanticDependenciesMissingError
 from basic_memory.schemas.search import SearchItemType, SearchRetrievalMode
 
+# sqlite-vec enforces a hard upper limit on the k parameter in knn queries.
+# Exceeding this limit raises OperationalError: k value in knn query too large.
+SQLITE_VEC_MAX_K = 4096
+
 
 class SQLiteSearchRepository(SearchRepositoryBase):
     """SQLite FTS5 implementation of search repository.
@@ -444,6 +448,9 @@ class SQLiteSearchRepository(SearchRepositoryBase):
         query_embedding: list[float],
         candidate_limit: int,
     ) -> list[dict]:
+        # sqlite-vec rejects k values above SQLITE_VEC_MAX_K with an OperationalError.
+        # Cap here so large projects (>4096 chunks) don't crash vector search.
+        candidate_limit = min(candidate_limit, SQLITE_VEC_MAX_K)
         query_embedding_json = json.dumps(query_embedding)
         vector_result = await session.execute(
             text(