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_exchange.py

@@ -20,6 +20,7 @@ import httpx
 import jwt
 
 from ..config import get_settings
+from ..http import nextcloud_httpx_client
 from .storage import RefreshTokenStorage
 
 logger = logging.getLogger(__name__)
@@ -68,7 +69,7 @@ class TokenExchangeService:
         self.storage: Optional[RefreshTokenStorage] = None
 
         # Create HTTP client
-        self.http_client = httpx.AsyncClient(
+        self.http_client = nextcloud_httpx_client(
             timeout=30.0,
             follow_redirects=True,
         )