fix: move audience mapper from scope to nextcloud-mcp-server client

The token-exchange-nextcloud scope was being inherited by DCR clients
and requested by external MCP clients (like Gemini CLI), causing all
tokens to have aud: "nextcloud" even when targeting the MCP server.

## Problem

When external clients registered via DCR, they inherited all optional
scopes from the realm defaults, including token-exchange-nextcloud. When
these clients requested tokens, they would include this scope, which added
aud: "nextcloud" via the scope's protocol mapper.

This caused authentication failures for MCP server access:
```
'aud': 'nextcloud'
WARNING - Token rejected: wrong audience ['nextcloud'], expected nextcloud-mcp-server
```

## Root Cause

Client scopes with protocol mappers are applied whenever that scope is
requested, regardless of which client requests it. The token-exchange-nextcloud
scope was designed for the MCP server's own token requests to Nextcloud APIs,
but external clients were also requesting it.

## Solution

Move the audience mapper from the token-exchange-nextcloud scope to a
direct protocol mapper on the nextcloud-mcp-server client itself.

### Changes

1. **Remove token-exchange-nextcloud from nextcloud-mcp-server optional scopes**
   - External DCR clients won't inherit this scope
   - Prevents external clients from requesting it

2. **Add nextcloud-audience protocol mapper directly to nextcloud-mcp-server**
   - Hardcode aud: "nextcloud" for this client only
   - Only tokens issued TO nextcloud-mcp-server will have this audience
   - External MCP clients won't be affected

## Behavior After Fix

**Gemini CLI (DCR client) → MCP Server**:
- Client doesn't have token-exchange-nextcloud scope
- Token audience: Based on RFC 8707 resource parameter (if provided)
- Result: No hardcoded audience ✅

**MCP Server (nextcloud-mcp-server) → Nextcloud APIs**:
- Client has nextcloud-audience protocol mapper
- Token audience: Always "nextcloud" (hardcoded)
- Result: aud: "nextcloud" for Nextcloud API access ✅

## Related
- RFC 8707: Resource Indicators for OAuth 2.0
- Keycloak client scopes vs. client protocol mappers
- DCR client scope inheritance

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

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

keycloak/realm-export.json

@@ -229,6 +229,18 @@
       "fullScopeAllowed": true,
       "nodeReRegistrationTimeout": -1,
       "protocolMappers": [
+        {
+          "name": "nextcloud-audience",
+          "protocol": "openid-connect",
+          "protocolMapper": "oidc-audience-mapper",
+          "consentRequired": false,
+          "config": {
+            "included.client.audience": "nextcloud",
+            "access.token.claim": "true",
+            "id.token.claim": "false",
+            "introspection.token.claim": "true"
+          }
+        },
         {
           "name": "sub",
           "protocol": "openid-connect",
@@ -308,7 +320,6 @@
         "phone",
         "offline_access",
         "microprofile-jwt",
-        "token-exchange-nextcloud",
         "notes:read",
         "notes:write",
         "calendar:read",