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/unified_verifier.py

@@ -31,6 +31,8 @@ from nextcloud_mcp_server.observability.metrics import (
     record_oauth_token_validation,
 )
 
+from ..http import nextcloud_httpx_client
+
 logger = logging.getLogger(__name__)
 
 
@@ -61,7 +63,7 @@ class UnifiedTokenVerifier(TokenVerifier):
         self.mode = "exchange" if settings.enable_token_exchange else "multi-audience"
 
         # Common components for all modes
-        self.http_client = httpx.AsyncClient(timeout=10.0)
+        self.http_client = nextcloud_httpx_client(timeout=10.0)
 
         # JWT verification support
         self.jwks_client: PyJWKClient | None = None