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/discord.provider.js

@@ -0,0 +1,30 @@
+// Built-in Discord provider (OAuth2). Endpoints hardcoded — admins configure only
+// Enabled + Client ID + Client Secret. `identify` yields the stable user id;
+// `email` yields the address. Discord's id is the stable per-user subject.
+
+const OAuth2Provider = require('./oauth2.provider')
+
+class DiscordProvider extends OAuth2Provider {
+  constructor(config = {}) {
+    super({ kind: 'discord', name: 'Discord', ...config, id: config.id || 'discord' })
+  }
+
+  authEndpoint() {
+    return 'https://discord.com/oauth2/authorize'
+  }
+  tokenEndpoint() {
+    return 'https://discord.com/api/oauth2/token'
+  }
+  userinfoEndpoint() {
+    return 'https://discord.com/api/users/@me'
+  }
+  scopeString() {
+    return 'identify email'
+  }
+  normalizeProfile(p = {}) {
+    // global_name is the new display name; fall back to the legacy username.
+    return { subject: p.id, email: p.email || null, name: p.global_name || p.username || null }
+  }
+}
+
+module.exports = DiscordProvider