- Removed trailing slash in '_oidc_client_issuer_url' to avoid issuer mismatch
- Use '.rstrip('/')' to normalize '_oidc_url'
- Switched to '~' concatenation instead of inline slashes for all OIDC endpoints
- Ensures that Mastodon and other OIDC clients match the issuer from Keycloak discovery
Change motivated by Mastodon issuer mismatch bug (OpenIDConnect::Discovery::DiscoveryFailed).
See related discussion: https://chatgpt.com/share/68a17d3c-c980-800f-934c-d56955b45f81