refactor(config): consolidate NEXTCLOUD_PUBLIC_ISSUER_URL through Settings

Lift NEXTCLOUD_PUBLIC_ISSUER_URL out of raw os.getenv reads into
Settings.nextcloud_public_issuer_url across all 8 production call sites
(app.py x2, oauth_routes.py x2, browser_oauth_routes.py,
provision_routes.py, userinfo_routes.py, elicitation.py). cli.py
remains the env-write source so the existing config-by-flag pipeline
still works.

Also addresses remaining PR #757 review nits:
- elicitation.py: align URL-present/absent wording on "open in your
  browser" so users don't try clicking in the terminal
- test_scope_authorization_stored.py: lock in the deliberately-shared
  fall-through branch with explicit declined/cancelled decorator tests
- test_elicitation.py: switch from monkeypatch.setenv to
  patch(get_settings) since Settings is now the canonical surface

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/provision_routes.py

@@ -15,7 +15,6 @@ Flow:
 
 import html
 import logging
-import os
 import secrets
 import time
 from urllib.parse import urlparse
@@ -251,7 +250,7 @@ async def provision_page(
     # LoginFlowV2Client) while login_url is rewritten to the public issuer
     # URL here because the browser needs a publicly-reachable address.
     login_url = init_response.login_url
-    public_issuer = os.getenv("NEXTCLOUD_PUBLIC_ISSUER_URL", "")
+    public_issuer = settings.nextcloud_public_issuer_url or ""
     if public_issuer and nextcloud_host:
         login_url = rewrite_url_origin(login_url, public_issuer.rstrip("/"))