feat: deliver schema 15 monthly settlement lifecycle

tests/test_monthly_settlement_creation.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+import sqlite3
+
+from jarca_ledger_mcp.application.monthly_settlement_service import create_monthly_settlement
+from jarca_ledger_mcp.infrastructure.sqlite.migrations import init_db
+
+
+def test_create_monthly_settlement_persists_canonical_draft_and_rejects_duplicate(tmp_path):
+    path = tmp_path / "ledger.sqlite3"
+    init_db(str(path))
+
+    created = create_monthly_settlement("2026-10", db_path=str(path))
+    assert created["success"]
+    assert created["month"] == "2026-10"
+    assert created["status"] == "draft"
+    assert created["settlement_id"]
+
+    with sqlite3.connect(path) as conn:
+        row = conn.execute(
+            "SELECT id, month, basis_date, status FROM monthly_settlement WHERE id=?",
+            (created["settlement_id"],),
+        ).fetchone()
+    assert row == (created["settlement_id"], "2026-10", "2026-10-31", "draft")
+
+    duplicate = create_monthly_settlement("2026-10", db_path=str(path))
+    assert duplicate["success"] is False
+    assert duplicate["error_code"] == "duplicate_month"
+    assert duplicate["settlement_id"] == created["settlement_id"]