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>

server/src/middleware/rateLimit.js

@@ -32,4 +32,23 @@ const contactLimiter = makeLimiter({
   message: 'Too many messages sent. Please try again later.',
 })
 
-module.exports = { loginLimiter, contactLimiter }
+// Cap mobile refresh-token exchanges per IP. Legitimate apps refresh at most a
+// handful of times per window; a flood is either a bug or an attempt to brute
+// the refresh endpoint.
+const mobileRefreshLimiter = makeLimiter({
+  windowMs: 15 * 60 * 1000,
+  max: 30,
+  label: 'mobile-refresh',
+  message: 'Too many refresh attempts. Please try again later.',
+})
+
+// Throttle SSO redirect starts per IP — cheap to trigger, and a flood is either a
+// bug or an attempt to spin the OAuth flow. Generous enough for real users.
+const ssoStartLimiter = makeLimiter({
+  windowMs: 15 * 60 * 1000,
+  max: 30,
+  label: 'sso-start',
+  message: 'Too many sign-in attempts. Please try again later.',
+})
+
+module.exports = { loginLimiter, contactLimiter, mobileRefreshLimiter, ssoStartLimiter }