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/oauth_routes.py

@@ -27,7 +27,6 @@ import time
 from base64 import urlsafe_b64encode
 from urllib.parse import urlencode
 
-import httpx
 import jwt
 from starlette.requests import Request
 from starlette.responses import JSONResponse, RedirectResponse
@@ -35,6 +34,8 @@ from starlette.responses import JSONResponse, RedirectResponse
 from nextcloud_mcp_server.auth.client_registry import get_client_registry
 from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
 
+from ..http import nextcloud_httpx_client
+
 logger = logging.getLogger(__name__)
 
 
@@ -218,7 +219,7 @@ async def oauth_authorize(request: Request) -> RedirectResponse | JSONResponse:
             )
 
         # Fetch authorization endpoint from discovery
-        async with httpx.AsyncClient() as http_client:
+        async with nextcloud_httpx_client() as http_client:
             response = await http_client.get(discovery_url)
             response.raise_for_status()
             discovery = response.json()
@@ -354,7 +355,7 @@ async def oauth_authorize_nextcloud(
             status_code=500,
         )
 
-    async with httpx.AsyncClient() as http_client:
+    async with nextcloud_httpx_client() as http_client:
         response = await http_client.get(discovery_url)
         response.raise_for_status()
         discovery = response.json()
@@ -462,7 +463,7 @@ async def oauth_callback_nextcloud(request: Request):
     callback_uri = f"{mcp_server_url}/oauth/callback"
 
     discovery_url = oauth_config.get("discovery_url")
-    async with httpx.AsyncClient() as http_client:
+    async with nextcloud_httpx_client() as http_client:
         response = await http_client.get(discovery_url)
         response.raise_for_status()
         discovery = response.json()
@@ -482,7 +483,7 @@ async def oauth_callback_nextcloud(request: Request):
         token_params["code_verifier"] = code_verifier
 
     # Exchange code for tokens
-    async with httpx.AsyncClient() as http_client:
+    async with nextcloud_httpx_client() as http_client:
         response = await http_client.post(
             token_endpoint,
             data=token_params,