feat: implement batch_store endpoint (Issue #12)

- Add batch_store tool accepting 1-50 entries per call
- Single DB transaction for atomicity
- Returns individual IDs/status per entry
- Add batch_store_memories() to Database layer
- Add 6 test cases
- Backward compatible - existing store unchanged

Expected impact: 50-60% reduction in store API calls

tests/e2e_mcp.rs

@@ -871,3 +871,83 @@ async fn e2e_auth_enabled_accepts_test_key() {
     let _ = server.kill();
     let _ = server.wait();
 }
+
+
+// =============================================================================
+// Batch Store Tests (Issue #12)
+// =============================================================================
+
+#[tokio::test]
+async fn e2e_batch_store_basic() -> anyhow::Result<()> {
+    let agent = format!("batch_{}", uuid::Uuid::new_v4());
+    let _ = db.purge_memories(&agent, None).await;
+
+    let resp = client.call_tool("batch_store", serde_json::json!({
+        "agent_id": agent.clone(),
+        "entries": [
+            {"content": "Fact alpha for batch test"},
+            {"content": "Fact beta for batch test"},
+            {"content": "Fact gamma for batch test"}
+        ]
+    })).await?;
+
+    let result: Value = serde_json::from_str(&resp.content[0].text)?;
+    assert!(result["success"].as_bool().unwrap_or(false));
+    assert_eq!(result["count"].as_i64().unwrap_or(0), 3);
+
+    db.purge_memories(&agent, None).await?;
+    Ok(())
+}
+
+#[tokio::test]
+async fn e2e_batch_store_empty_rejected() -> anyhow::Result<()> {
+    let resp = client.call_tool("batch_store", serde_json::json!({
+        "entries": []
+    })).await;
+    assert!(resp.is_err() || resp.as_ref().unwrap().is_error());
+    Ok(())
+}
+
+#[tokio::test]
+async fn e2e_batch_store_exceeds_max() -> anyhow::Result<()> {
+    let entries: Vec<Value> = (0..51).map(|i| serde_json::json!({"content": format!("Entry {}", i)})).collect();
+    let resp = client.call_tool("batch_store", serde_json::json!({
+        "entries": entries
+    })).await;
+    assert!(resp.is_err() || resp.as_ref().unwrap().is_error());
+    Ok(())
+}
+
+#[tokio::test]
+async fn e2e_batch_store_missing_content() -> anyhow::Result<()> {
+    let resp = client.call_tool("batch_store", serde_json::json!({
+        "entries": [{"content": "Valid entry"}, {"metadata": {}}]
+    })).await;
+    assert!(resp.is_err() || resp.as_ref().unwrap().is_error());
+    Ok(())
+}
+
+#[tokio::test]
+async fn e2e_batch_store_appears_in_tools() -> anyhow::Result<()> {
+    let tools = client.list_tools().await?;
+    let parsed: Value = serde_json::from_str(&tools.content[0].text)?;
+    let names: Vec<&str> = parsed.as_array().unwrap().iter()
+        .filter_map(|t| t.get("name").and_then(|n| n.as_str()))
+        .collect();
+    assert!(names.contains(&"batch_store"));
+    Ok(())
+}
+
+#[tokio::test]
+async fn e2e_existing_store_unchanged() -> anyhow::Result<()> {
+    let agent = format!("compat_{}", uuid::Uuid::new_v4());
+    let _ = db.purge_memories(&agent, None).await;
+    let resp = client.call_tool("store", serde_json::json!({
+        "agent_id": agent.clone(),
+        "content": "Original store still works"
+    })).await?;
+    let result: Value = serde_json::from_str(&resp.content[0].text)?;
+    assert!(result["success"].as_bool().unwrap_or(false));
+    db.purge_memories(&agent, None).await?;
+    Ok(())
+}