fix: Use NEXTCLOUD_OIDC_CLIENT_ID/SECRET env vars consistently

Fixes #296

The application code was looking for OIDC_CLIENT_ID and OIDC_CLIENT_SECRET
(without NEXTCLOUD_ prefix), but the Helm chart, documentation, and CLI
all use NEXTCLOUD_OIDC_CLIENT_ID and NEXTCLOUD_OIDC_CLIENT_SECRET.

This mismatch caused OAuth deployments via Helm to fail with crashloops
because the credentials weren't being found.

Changes:
- app.py: Use NEXTCLOUD_OIDC_CLIENT_ID/SECRET in setup_oauth_config()
- config.py: Use NEXTCLOUD_OIDC_CLIENT_ID/SECRET in get_settings()
- Updated documentation comments and error messages

This aligns with the documented naming convention where all Nextcloud-related
environment variables use the NEXTCLOUD_ prefix.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/config.py

@@ -288,8 +288,8 @@ def get_settings() -> Settings:
     return Settings(
         # OAuth/OIDC settings
         oidc_discovery_url=os.getenv("OIDC_DISCOVERY_URL"),
-        oidc_client_id=os.getenv("OIDC_CLIENT_ID"),
-        oidc_client_secret=os.getenv("OIDC_CLIENT_SECRET"),
+        oidc_client_id=os.getenv("NEXTCLOUD_OIDC_CLIENT_ID"),
+        oidc_client_secret=os.getenv("NEXTCLOUD_OIDC_CLIENT_SECRET"),
         oidc_issuer=os.getenv("OIDC_ISSUER"),
         # Nextcloud settings
         nextcloud_host=os.getenv("NEXTCLOUD_HOST"),