Add displayName to auth response and configurable display name claim for OIDC

- Add displayName field to AuthTokenResponse so the UI shows human-readable
  names instead of internal JWT subjects (e.g. user:oidc:<hash>)
- Add displayNameClaim to OIDC config (default: "name") allowing admins to
  configure which ID token claim contains the user's display name
- Support dot-separated claim paths (e.g. profile.display_name) like rolesClaim
- Add admin UI field for Display Name Claim on the OIDC config page
- ClickHouse migration: ALTER TABLE adds display_name_claim column

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clickhouse/init/06-oidc-display-name-claim.sql

@@ -0,0 +1 @@
+ALTER TABLE oidc_config ADD COLUMN IF NOT EXISTS display_name_claim String DEFAULT 'name';