feat: Add rate limiting and extract helpers for app password endpoints

Security improvements:
- Add in-memory rate limiter for app password provisioning (5 attempts/hour/user)
- Returns 429 Too Many Requests with Retry-After header when limit exceeded
- Rate limiting is per-user to prevent cross-user DoS

Code quality improvements:
- Extract _extract_basic_auth() helper to reduce duplication across 3 endpoints
- Move base64, re imports to module level
- Add APP_PASSWORD_PATTERN constant for regex validation
- Add NEXTCLOUD_VALIDATION_TIMEOUT constant (10s)

Test coverage:
- Add test_provision_app_password_rate_limiting
- Add test_rate_limiting_is_per_user
- Add autouse fixture to clear rate limit state between tests
- Total: 15 tests for management API endpoints

Addresses reviewer feedback on PR #473.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/test_management_app_password_endpoints.py

@@ -18,6 +18,7 @@ from starlette.applications import Starlette
 from starlette.routing import Route
 from starlette.testclient import TestClient
 
+from nextcloud_mcp_server.api import management
 from nextcloud_mcp_server.api.management import (
     delete_app_password,
     get_app_password_status,
@@ -28,6 +29,14 @@ from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
 pytestmark = pytest.mark.unit
 
 
+@pytest.fixture(autouse=True)
+def clear_rate_limit():
+    """Clear rate limit state before each test."""
+    management._rate_limit_attempts.clear()
+    yield
+    management._rate_limit_attempts.clear()
+
+
 @pytest.fixture
 def encryption_key():
     """Generate a test encryption key."""
@@ -487,3 +496,129 @@ async def test_delete_app_password_username_mismatch():
     )
 
     assert response.status_code == 403
+
+
+async def test_provision_app_password_rate_limiting(mocker):
+    """Test that rate limiting blocks excessive provisioning attempts."""
+    mocker.patch(
+        "nextcloud_mcp_server.config.get_settings",
+        return_value=MagicMock(nextcloud_host="http://localhost:8080"),
+    )
+
+    # Mock httpx client to return 401 (failed validation)
+    mock_response = MagicMock()
+    mock_response.status_code = 401
+
+    mock_client = AsyncMock()
+    mock_client.get = AsyncMock(return_value=mock_response)
+    mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+    mock_client.__aexit__ = AsyncMock()
+
+    mocker.patch(
+        "nextcloud_mcp_server.api.management.httpx.AsyncClient",
+        return_value=mock_client,
+    )
+
+    app = Starlette(
+        routes=[
+            Route(
+                "/api/v1/users/{user_id}/app-password",
+                provision_app_password,
+                methods=["POST"],
+            ),
+        ]
+    )
+
+    client = TestClient(app)
+
+    # Make 5 failed attempts (should all return 401)
+    for i in range(5):
+        response = client.post(
+            "/api/v1/users/testuser/app-password",
+            headers={
+                "Authorization": create_basic_auth_header(
+                    "testuser", "aaaaa-bbbbb-ccccc-ddddd-eeeee"
+                )
+            },
+        )
+        assert response.status_code == 401, f"Attempt {i + 1} should return 401"
+
+    # 6th attempt should be rate limited (429)
+    response = client.post(
+        "/api/v1/users/testuser/app-password",
+        headers={
+            "Authorization": create_basic_auth_header(
+                "testuser", "aaaaa-bbbbb-ccccc-ddddd-eeeee"
+            )
+        },
+    )
+    assert response.status_code == 429
+    assert "Rate limit exceeded" in response.json()["error"]
+    assert "Retry-After" in response.headers
+
+
+async def test_rate_limiting_is_per_user(mocker):
+    """Test that rate limiting is applied per user, not globally."""
+    mocker.patch(
+        "nextcloud_mcp_server.config.get_settings",
+        return_value=MagicMock(nextcloud_host="http://localhost:8080"),
+    )
+
+    # Mock httpx client to return 401
+    mock_response = MagicMock()
+    mock_response.status_code = 401
+
+    mock_client = AsyncMock()
+    mock_client.get = AsyncMock(return_value=mock_response)
+    mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+    mock_client.__aexit__ = AsyncMock()
+
+    mocker.patch(
+        "nextcloud_mcp_server.api.management.httpx.AsyncClient",
+        return_value=mock_client,
+    )
+
+    app = Starlette(
+        routes=[
+            Route(
+                "/api/v1/users/{user_id}/app-password",
+                provision_app_password,
+                methods=["POST"],
+            ),
+        ]
+    )
+
+    client = TestClient(app)
+
+    # Make 5 failed attempts for user1 (hits rate limit)
+    for _ in range(5):
+        client.post(
+            "/api/v1/users/user1/app-password",
+            headers={
+                "Authorization": create_basic_auth_header(
+                    "user1", "aaaaa-bbbbb-ccccc-ddddd-eeeee"
+                )
+            },
+        )
+
+    # user1 should be rate limited
+    response = client.post(
+        "/api/v1/users/user1/app-password",
+        headers={
+            "Authorization": create_basic_auth_header(
+                "user1", "aaaaa-bbbbb-ccccc-ddddd-eeeee"
+            )
+        },
+    )
+    assert response.status_code == 429
+
+    # user2 should NOT be rate limited (different user)
+    response = client.post(
+        "/api/v1/users/user2/app-password",
+        headers={
+            "Authorization": create_basic_auth_header(
+                "user2", "bbbbb-ccccc-ddddd-eeeee-fffff"
+            )
+        },
+    )
+    assert response.status_code == 401  # Fails validation, but not rate limited