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/router/v1/auth/sso.routes.js

@@ -0,0 +1,22 @@
+const express = require('express')
+
+const ctrl = require('./sso.controller')
+const { requireAuth } = require('../../../auth/session.middleware')
+const { ssoStartLimiter } = require('../../../middleware/rateLimit')
+
+const ssoRouter = express.Router()
+
+// Public discovery — the login page reads this to render provider buttons.
+ssoRouter.get('/providers', ctrl.listProviders)
+
+// Begin login (public) — redirects to the IdP.
+ssoRouter.get('/sso/:provider/start', ssoStartLimiter, ctrl.start)
+
+// Begin account linking (must be signed in — the tx captures the acting user).
+ssoRouter.get('/sso/:provider/link', requireAuth, ctrl.linkStart)
+
+// OAuth redirect target — completes login or linking. Not behind requireAuth:
+// the signed tx cookie authorizes link mode; login mode is link-only anyway.
+ssoRouter.get('/sso/:provider/callback', ctrl.callback)
+
+module.exports = ssoRouter