feat: add self-signed SSL certificate support for Nextcloud connections

Add NEXTCLOUD_VERIFY_SSL and NEXTCLOUD_CA_BUNDLE env vars to configure
TLS certificate verification for all outbound Nextcloud connections.
Centralizes SSL config via a new HTTP client factory (http.py) used by
all 27 Nextcloud-bound call sites, including API clients, OIDC endpoints,
OAuth flows, and health checks.

Closes #560

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nextcloud_mcp_server/auth/token_broker.py

@@ -25,6 +25,8 @@ import jwt
 from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
 from nextcloud_mcp_server.auth.token_exchange import exchange_token_for_delegation
 
+from ..http import nextcloud_httpx_client
+
 logger = logging.getLogger(__name__)
 
 
@@ -136,7 +138,7 @@ class TokenBrokerService:
     async def _get_http_client(self) -> httpx.AsyncClient:
         """Get or create HTTP client."""
         if self._http_client is None:
-            self._http_client = httpx.AsyncClient(
+            self._http_client = nextcloud_httpx_client(
                 timeout=httpx.Timeout(30.0), follow_redirects=True
             )
         return self._http_client