fix(docs): correct OIDC callback URL to use OIDC_NAME

[krokosik]

Description

The docs hardcoded /api/auth/callback/oidc, but the actual OIDC provider id is env.OIDC_NAME?.toLowerCase() ?? 'oidc' (src/server/auth.ts:249). Users setting OIDC_NAME=tinyauth therefore need /api/auth/callback/tinyauth, and the previous docs led them to configure the wrong redirect URI in their IdP.

Updated docs/AUTHENTICATION.md to document the parameterized form and include an example.

Closes #679

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• The last commit successfully passed pre-commit checks

Note: pnpm tsgo --noEmit currently fails on main due to a pre-existing error in src/components/group/AddMembers.tsx:89 (sendInviteEmail not in tRPC input type), unrelated to this docs-only change.