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/App.jsx

@@ -27,6 +27,7 @@ import HeroEditor from './routes/admin/views/HeroEditor.jsx'
 import SettingsAdmin from './routes/admin/views/SettingsAdmin.jsx'
 import ActivityAdmin from './routes/admin/views/ActivityAdmin.jsx'
 import BotActivityAdmin from './routes/admin/views/BotActivityAdmin.jsx'
+import AuthProvidersAdmin from './routes/admin/views/AuthProvidersAdmin.jsx'
 import UsersAdmin from './routes/admin/views/UsersAdmin.jsx'
 import AccountAdmin from './routes/admin/views/AccountAdmin.jsx'
 
@@ -73,6 +74,7 @@ export default function App() {
             <Route path="settings" element={<SettingsAdmin />} />
             <Route path="activity" element={<ActivityAdmin />} />
             <Route path="bot-activity" element={<BotActivityAdmin />} />
+            <Route path="auth-providers" element={<AuthProvidersAdmin />} />
             <Route path="users" element={<UsersAdmin />} />
             <Route path="account" element={<AccountAdmin />} />
             <Route path="*" element={<Navigate to="/admin" replace />} />