* feat: make upstream_client_secret optional in OAuthProxy
Extract _create_upstream_oauth_client() factory method for subclass
override. Cookie signing falls back to JWT key material when no secret.
* fix: include client_id in revocation requests for public clients
* fix: use factory method for revocation auth
* Add Discord OAuth provider and corresponding tests
* Update DiscordProvider client_secret and required_scopes documentation
* Add Discord to authentication support list in README
* Fix Discord token verifier to match actual API response format
Discord's /api/oauth2/@me endpoint returns:
- "scopes" as a list, not "scope" as a space-separated string
- "expires" as ISO timestamp, not "expires_in" as seconds
- "user" data directly in the response (no need for extra API call)
* Simplify Discord token verifier
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Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>