fix: Implement proper OAuth resource parameters and PRM-based discovery

This commit completes the OAuth audience validation implementation per RFC 7519,
RFC 8707 (Resource Indicators), and RFC 9728 (Protected Resource Metadata).

## Key Changes

### OAuth Resource Parameters (RFC 8707)
- Add `resource` parameter to Flow 1 (MCP client auth) with MCP server audience
- Add `resource` parameter to Flow 2 (Nextcloud access) with Nextcloud audience
- Add `nextcloud_resource_uri` to oauth_context configuration
- Fix undefined variable error in starlette_lifespan

### PRM-Based Resource Discovery (RFC 9728)
- Update tests to fetch resource identifier from PRM endpoint
- Add fallback to hardcoded value if PRM fetch fails
- Demonstrate correct OAuth client implementation pattern

### ADR-005 Documentation Updates
- Update to reflect simplified RFC 7519 compliant implementation
- Document that MCP validates only its own audience (not Nextcloud's)
- Add section on OAuth resource parameters and PRM discovery
- Update implementation checklist to show completed items
- Mark status as "Implemented" with update date

## Implementation Details

The solution follows RFC 7519 Section 4.1.3: resource servers validate only
their own presence in the audience claim. This simplifies the logic while
maintaining security:

- MCP server validates MCP audience only
- Nextcloud independently validates its own audience
- No dual validation required at MCP layer
- Token reuse is allowed per RFC 8707 for multi-audience tokens

## Test Results
✅ test_mcp_oauth_server_connection - PASSED
✅ test_deck_board_view_permissions - PASSED
✅ test_prm_endpoint - PASSED

All OAuth flows now properly specify target resources, resulting in correct
audience validation throughout the system.

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

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

nextcloud_mcp_server/auth/oauth_routes.py

@@ -252,6 +252,7 @@ async def oauth_authorize(request: Request) -> RedirectResponse | JSONResponse:
             "scope": scopes,
             "state": idp_state,
             "prompt": "consent",  # Ensure refresh token
+            "resource": f"{oauth_config['mcp_server_url']}/mcp",  # MCP server audience
         }
 
         auth_url = f"{authorization_endpoint}?{urlencode(idp_params)}"
@@ -359,6 +360,7 @@ async def oauth_authorize_nextcloud(
         "state": state,
         "prompt": "consent",  # Force consent to show resource access
         "access_type": "offline",  # Request refresh token
+        "resource": oauth_config["nextcloud_resource_uri"],  # Nextcloud audience
     }
 
     auth_url = f"{authorization_endpoint}?{urlencode(idp_params)}"