Add session abstraction, mobile bearer auth, and pluggable SSO

Refactor authentication into a provider-agnostic session layer and build
two new auth surfaces on top of it, without changing local password/TOTP
behavior. Every flow now issues sessions through
sessionService.createSession(user, authMethod).

Part 1 — Session abstraction (backward-compatible refactor):
- New server/src/auth/: token.js (JWT/cookie primitives), session.service.js
  (create/validate/partial-TOTP/revoke), session.middleware.js
  (attachSession/requireAuth/requireRole). utils/auth.js is now a thin
  compat facade so existing imports are unchanged.

Part 2 — Mobile bearer auth (additive):
- /api/v1/auth/mobile/{login,refresh,logout}: short-lived access JWT +
  long-lived refresh token, stored hashed and rotated on use, in a new
  mobile_refresh_tokens table. Reuses web bot-scoring/backoff; single-request
  TOTP. token.signToken gains a backward-compatible expiresIn option.

Part 3 — Pluggable SSO (Google, Discord, generic OIDC):
- OAuth2Provider base + built-in Google/Discord (fixed endpoints) + generic
  OIDC, a registry with health/validation, PKCE+CSRF transaction state, and
  discovery (GET /auth/providers), start/link/callback routes.
- Link-only policy: SSO signs in only to an already-linked account; external
  identities are never auto-provisioned. Client secrets encrypted at rest
  (AES-256-GCM, utils/secretBox.js). Admin CRUD (/admin/auth/providers) and
  account linking (/admin/account/identities). New auth_providers +
  user_identities tables.

Frontend:
- Login page renders provider buttons from /auth/providers (inline SVG icons,
  graceful with zero providers). New Authentication admin view
  (Local/Google/Discord/Custom). Account page linked-accounts section.

Tests: 83 passing (session, mobile, providers, registry, secretBox, ssoState,
ssoCallback) — all DB-free via fetch mocks + model stubs. README + .env.example
updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/src/api/client.js

@@ -47,6 +47,8 @@ export const api = {
   loginTotp: (challenge, code) =>
     req('/auth/login/totp', { method: 'POST', body: { challenge, code } }),
   logout: () => req('/auth/logout', { method: 'POST' }),
+  // Public SSO provider discovery — drives the login-page provider buttons.
+  authProviders: () => req('/auth/providers'),
 
   // ----- public -----
   publicSettings: () => req('/public/settings'),
@@ -120,6 +122,16 @@ export const api = {
     totpSetup: () => req('/admin/account/totp/setup', { method: 'POST' }),
     totpEnable: (code) => req('/admin/account/totp/enable', { method: 'POST', body: { code } }),
     totpDisable: (code) => req('/admin/account/totp/disable', { method: 'POST', body: { code } }),
+
+    // ----- linked SSO identities (self-service) -----
+    linkedIdentities: () => req('/admin/account/identities'),
+    unlinkIdentity: (provider) => req(`/admin/account/identities/${provider}`, { method: 'DELETE' }),
+
+    // ----- auth providers / SSO config (admin only) -----
+    listAuthProviders: () => req('/admin/auth/providers'),
+    createAuthProvider: (data) => req('/admin/auth/providers', { method: 'POST', body: data }),
+    updateAuthProvider: (id, data) => req(`/admin/auth/providers/${id}`, { method: 'PUT', body: data }),
+    deleteAuthProvider: (id) => req(`/admin/auth/providers/${id}`, { method: 'DELETE' }),
   },
 }