Reduce embedding batch_size from 16 to 2 to prevent llama-server crash

The bge-small-en-v1.5-f32.gguf model has n_ctx_train=512. With batch_size=16
and ~300+ tokens per chunk, total tokens exceed 512 causing GGML_ASSERT crash.

Now with batch_size=2, embeddings are processed safely.

src/core/kb/embedding_generator.rs

@@ -43,7 +43,7 @@ impl Default for EmbeddingConfig {
             embedding_model: "BAAI/bge-multilingual-gemma2".to_string(),
             embedding_key: None,
             dimensions: 2048,
-            batch_size: 16,
+            batch_size: 2,
             timeout_seconds: 60,
             max_concurrent_requests: 1,
             connect_timeout_seconds: 10,