fix: address PR review comments (round 2)

Human reviewer (zmanian):
- H1: Accept configurable embedding dimension in LanceDbVectorStore::new()
  instead of hardcoding 1536. Dimension is sourced from
  EmbeddingProvider::dimension() at init time.
- H2: Skip double-write of embeddings to DB when external vector store
  is active (pass None to insert_chunk for embedding column).
- H3: Update PR title from "refactor" to "feat" (net-new feature).
- H4: Document non-atomic update_embedding in struct doc comment.

Bot reviewer (Copilot):
- Cache LanceDB table handle via tokio::sync::OnceCell (avoid
  open_table per operation).
- Cache Arc<Schema> in struct (avoid rebuilding per insert).
- Fix error variants: ChunkingFailed → EmbeddingFailed for LanceDB
  store/delete operations.
- Propagate store_embedding errors in reindex_document instead of
  warn-only (prevents silent data loss).
- Prefetch document metadata map in backfill_embeddings to avoid N+1
  queries.
- Add lancedb feature + protoc to CI test matrix so LanceDB tests
  actually run on Linux.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

tests/lancedb_integration.rs

@@ -57,7 +57,7 @@ async fn setup_workspace() -> (Workspace, TempDir, TempDir) {
     libsql.run_migrations().await.unwrap();
 
     let lancedb_dir = TempDir::new().unwrap();
-    let store = LanceDbVectorStore::new(lancedb_dir.path()).await.unwrap();
+    let store = LanceDbVectorStore::new(lancedb_dir.path(), None).await.unwrap();
 
     let embedding = make_embedding(1.0);
     let ws = Workspace::new_with_db("test_user", Arc::new(libsql) as Arc<dyn Database>)