API key come hash SHA-256, una per sessione, TTL rinnovabile (#41)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/backend/tests/test_user_password_endpoint.py

@@ -17,7 +17,7 @@ class TestChangeUserPassword:
 
         the same as setup.py's change-user-password (setup.py:634).
         """
-        old_key = maker_user.api_key
+        old_key = maker_user.plain_key
         old_username = maker_user.username
         assert old_key is not None
 
@@ -31,7 +31,7 @@ class TestChangeUserPassword:
 
         # The old key no longer authenticates. Read it before the request
         # above: the client and app share one session (see conftest), so
-        # `maker_user.api_key` in this process reflects the mutation too.
+        # `maker_user.plain_key` in this process reflects the mutation too.
         resp2 = await client.get("/api/auth/me", headers={"X-API-Key": old_key})
         assert resp2.status_code == 401