feat(auth): add multi-user BasicAuth pass-through mode

Implement multi-user BasicAuth pass-through mode (ADR-020) where each
request includes BasicAuth credentials that are forwarded to Nextcloud
APIs without persistent storage.

Changes:
- Add _get_client_from_basic_auth() in context.py to extract credentials
  from Authorization header (set by BasicAuthMiddleware)
- Add AstrolabeClient for app password provisioning via Astrolabe API
- Update oauth_sync.py with dual credential support (app passwords first,
  then refresh tokens as fallback)
- Simplify oauth_tools.py provisioning logic
- Add integration tests for app password provisioning and multi-user BasicAuth

Features:
- Stateless multi-user mode: credentials passed per-request
- Optional background sync via app passwords (stored in Astrolabe)
- Falls back to refresh tokens if app password not available
- Test coverage for provisioning flow and pass-through mode

Related: ADR-019 (Multi-user BasicAuth), ADR-020 (Deployment Modes)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nextcloud_mcp_server/auth/astrolabe_client.py

@@ -0,0 +1,152 @@
+"""
+Client for querying Astrolabe Management API for background sync credentials.
+
+This client uses OAuth client credentials flow to authenticate to Nextcloud
+and retrieve user app passwords for background sync operations.
+"""
+
+import logging
+import time
+from typing import Optional
+
+import httpx
+
+logger = logging.getLogger(__name__)
+
+
+class AstrolabeClient:
+    """Client for querying Astrolabe API for background sync credentials.
+
+    Uses OAuth client credentials flow to authenticate as the MCP server
+    and retrieve user app passwords that are stored in Nextcloud.
+    """
+
+    def __init__(
+        self,
+        nextcloud_host: str,
+        client_id: str,
+        client_secret: str,
+    ):
+        """
+        Initialize Astrolabe client.
+
+        Args:
+            nextcloud_host: Nextcloud base URL (e.g., https://cloud.example.com)
+            client_id: OAuth client ID for MCP server
+            client_secret: OAuth client secret
+        """
+        self.nextcloud_host = nextcloud_host.rstrip("/")
+        self.client_id = client_id
+        self.client_secret = client_secret
+        self._token_cache: Optional[dict] = None  # {access_token, expires_at}
+
+    async def get_access_token(self) -> str:
+        """
+        Get access token using OAuth client credentials flow.
+
+        Tokens are cached with 1-minute early refresh to avoid expiration.
+
+        Returns:
+            Access token string
+
+        Raises:
+            httpx.HTTPError: If token request fails
+        """
+        # Check cache
+        if self._token_cache and time.time() < self._token_cache["expires_at"]:
+            logger.debug("Using cached OAuth token for Astrolabe API")
+            return self._token_cache["access_token"]
+
+        # Discover token endpoint
+        discovery_url = f"{self.nextcloud_host}/.well-known/openid-configuration"
+
+        async with httpx.AsyncClient() as client:
+            logger.debug(f"Discovering token endpoint from {discovery_url}")
+            discovery_resp = await client.get(discovery_url)
+            discovery_resp.raise_for_status()
+            token_endpoint = discovery_resp.json()["token_endpoint"]
+
+            logger.debug(f"Requesting client credentials token from {token_endpoint}")
+
+            # Request token using client credentials grant
+            token_resp = await client.post(
+                token_endpoint,
+                data={
+                    "grant_type": "client_credentials",
+                    "client_id": self.client_id,
+                    "client_secret": self.client_secret,
+                    "scope": "openid",  # Minimal scope
+                },
+            )
+            token_resp.raise_for_status()
+            data = token_resp.json()
+
+            # Cache with 1-minute early refresh
+            expires_in = data.get("expires_in", 3600)
+            self._token_cache = {
+                "access_token": data["access_token"],
+                "expires_at": time.time() + expires_in - 60,
+            }
+
+            logger.info(f"Obtained Astrolabe API token (expires in {expires_in}s)")
+            return data["access_token"]
+
+    async def get_user_app_password(self, user_id: str) -> Optional[str]:
+        """
+        Retrieve user's app password for background sync.
+
+        Args:
+            user_id: Nextcloud user ID
+
+        Returns:
+            App password string, or None if user hasn't provisioned
+
+        Raises:
+            httpx.HTTPError: If API request fails (except 404)
+        """
+        token = await self.get_access_token()
+        url = f"{self.nextcloud_host}/apps/astrolabe/api/v1/background-sync/credentials/{user_id}"
+
+        async with httpx.AsyncClient() as client:
+            logger.debug(f"Retrieving app password for user: {user_id}")
+
+            response = await client.get(
+                url,
+                headers={"Authorization": f"Bearer {token}"},
+                timeout=10.0,
+            )
+
+            if response.status_code == 404:
+                logger.debug(f"No app password configured for user: {user_id}")
+                return None
+
+            response.raise_for_status()
+            data = response.json()
+
+            logger.info(
+                f"Retrieved app password for user: {user_id} (type: {data.get('credential_type')})"
+            )
+            return data.get("app_password")
+
+    async def get_background_sync_status(self, user_id: str) -> dict:
+        """
+        Get background sync status for a user.
+
+        Args:
+            user_id: Nextcloud user ID
+
+        Returns:
+            Dict with keys: has_access, credential_type, provisioned_at
+
+        Raises:
+            httpx.HTTPError: If API request fails
+        """
+        # For now, check if app password exists
+        # In the future, this could query a dedicated status endpoint
+        app_password = await self.get_user_app_password(user_id)
+
+        return {
+            "has_access": app_password is not None,
+            "credential_type": "app_password" if app_password else None,
+            "provisioned_at": None,  # TODO: Get from API if available
+        }