Implements three OIDC follow-ups from specs/portal/cluster-oidc.md:
- Zero-touch instance OIDC RP config: PHOTOPRISM_CLUSTER_OIDC derives the
OIDC RP client_id/secret from the node client credentials and defaults
the issuer to the instance's own origin (scheme.OriginURL), so a single
boot reaches a working Portal login. Explicit oidc-client/secret/uri win.
- Human-friendly authorize errors: redirectable errors 302 back to the
validated redirect_uri; non-redirectable ones content-negotiate a
branded HTML page (oauth-error.gohtml) vs JSON via shared helpers in
internal/api/oauth_error.go. The RP callback surfaces inbound ?error=.
- Cluster-wide Sign-Out (Tier 2): session.logoutEverywhere fans out a
best-effort DELETE per peer session, clears their storage, then signs
out locally; session-delete clears the OP cookie on any shared-domain
OIDC-against-Portal node (OIDCSessionCookieClearPath).
