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/auth/providers/google.provider.js

@@ -0,0 +1,34 @@
+// Built-in Google provider (OAuth2 / OpenID Connect). Endpoints are hardcoded —
+// admins configure only Enabled + Client ID + Client Secret. Uses the OIDC
+// userinfo endpoint; `sub` is Google's stable per-user id.
+
+const OAuth2Provider = require('./oauth2.provider')
+
+class GoogleProvider extends OAuth2Provider {
+  constructor(config = {}) {
+    super({ kind: 'google', name: 'Google', ...config, id: config.id || 'google' })
+  }
+
+  authEndpoint() {
+    return 'https://accounts.google.com/o/oauth2/v2/auth'
+  }
+  tokenEndpoint() {
+    return 'https://oauth2.googleapis.com/token'
+  }
+  userinfoEndpoint() {
+    return 'https://openidconnect.googleapis.com/v1/userinfo'
+  }
+  scopeString() {
+    return 'openid email profile'
+  }
+  authParams() {
+    // Online access (no refresh token needed for login), and let the user pick
+    // an account rather than silently reusing a signed-in one.
+    return { access_type: 'online', prompt: 'select_account' }
+  }
+  normalizeProfile(p = {}) {
+    return { subject: p.sub, email: p.email || null, name: p.name || p.email || null }
+  }
+}
+
+module.exports = GoogleProvider