feat: implement legacy history migration (closes #24)

src/fenris/store.py

@@ -166,6 +166,15 @@ def _create_schema(conn: sqlite3.Connection):
     """)
 
 
+    # Metadata table for store state (e.g., legacy import marker)
+    conn.execute("""
+        CREATE TABLE IF NOT EXISTS store_metadata (
+            key TEXT PRIMARY KEY,
+            value TEXT NOT NULL
+        )
+    """)
+
+
 def _apply_migrations(conn: sqlite3.Connection, current_version: int):
     """Apply forward-only migrations from current_version to SCHEMA_VERSION."""
     # Future migrations will go here