feat(auth): drop test-client defaults, add ALLOWED_MGMT_CLIENT allowlist

Both auth surfaces now fail-closed by default:

- ALLOWED_MCP_CLIENTS: removed the silent `claude-desktop` and
  `test-mcp-client` fallbacks. Empty/unset env var leaves the registry
  empty so /oauth/authorize rejects every client_id.
- ALLOWED_MGMT_CLIENT (new): comma-separated list of OIDC client_ids
  whose tokens are accepted by /api/management/*. Enforced in
  verify_token_for_management_api on both the cache-hit and cache-miss
  paths against the token's client_id claim. Unset/empty rejects all.

Compose: set ALLOWED_MGMT_CLIENT=nextcloudMcpServerUIPublicClient on
mcp-multi-user-basic so the existing Astrolabe integration test
(test_astrolabe_chunk_context.py) still passes.

env.sample documents both vars and notes they may be consolidated later.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -168,6 +168,11 @@ services:
       # - NEXTCLOUD_OIDC_CLIENT_ID=your_client_id
       # - NEXTCLOUD_OIDC_CLIENT_SECRET=your_client_secret
 
+      # Management API allowlist (ADR-018): only Astrolabe-issued tokens may
+      # call /api/management/*. Default test client_id used by the
+      # configure_astrolabe_for_mcp_server fixture.
+      - ALLOWED_MGMT_CLIENT=nextcloudMcpServerUIPublicClient
+
       # NO admin credentials - credentials come from client Authorization header
     volumes:
       - multi-user-basic-data:/app/data