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/webhook_routes.py

@@ -20,6 +20,8 @@ from nextcloud_mcp_server.server.webhook_presets import (
     get_preset,
 )
 
+from ..http import nextcloud_httpx_client
+
 logger = logging.getLogger(__name__)
 
 
@@ -140,7 +142,7 @@ async def _get_authenticated_client(request: Request) -> httpx.AsyncClient:
 
         assert nextcloud_host is not None  # Type narrowing for type checker
         assert username is not None and password is not None  # Type narrowing
-        return httpx.AsyncClient(
+        return nextcloud_httpx_client(
             base_url=nextcloud_host,
             auth=(username, password),
             timeout=30.0,
@@ -163,7 +165,7 @@ async def _get_authenticated_client(request: Request) -> httpx.AsyncClient:
     if not nextcloud_host:
         raise RuntimeError("Nextcloud host not configured")
 
-    return httpx.AsyncClient(
+    return nextcloud_httpx_client(
         base_url=nextcloud_host,
         headers={"Authorization": f"Bearer {access_token}"},
         timeout=30.0,