feat(api): policy and root CRUD endpoints

Policies and roots were editable only via SQL inside the container.
Full CRUD over both, validated against the vocabulary the policy
engine knows — unknown resolutions or sources answer 422 naming the
field, malformed JSON answers 422, deleting a referenced policy or an
occupied root answers 409. Closes #116 (API half).

.sqlx/query-0a27365a669aa217ae7dae675ebf54c4e4a343c4b56c4a44eb56d1a26c2aea4b.json

@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "UPDATE roots SET kind = ?, audience = ?, path = ?, policy_id = ?,\n               updated_at = strftime('%Y-%m-%dT%H:%M:%fZ', 'now')\n           WHERE id = ?",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 5
+    },
+    "nullable": []
+  },
+  "hash": "0a27365a669aa217ae7dae675ebf54c4e4a343c4b56c4a44eb56d1a26c2aea4b"
+}