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/model/userIdentities/userIdentities.model.js

@@ -0,0 +1,27 @@
+// Account-linking store: maps external SSO identities to internal users. Thin
+// logic layer over userIdentities.db (mirrors the users model split).
+
+const db = require('./userIdentities.db')
+
+// The link-only login lookup. Returns the identity row (with user_id) or null.
+async function findByProviderSubject(provider, subject) {
+  return db.findByProviderSubject(provider, subject)
+}
+
+// Identities linked to a user (Account page).
+async function listForUser(userId) {
+  return db.listForUser(userId)
+}
+
+// Link an external identity to an internal user. Returns the new row id. The
+// (provider, subject) UNIQUE constraint enforces one-identity-one-user at the DB.
+async function link({ userId, provider, subject, email }) {
+  return db.insert({ userId, provider, subject, email })
+}
+
+// Unlink a provider from a user. Returns rows removed (0 if nothing was linked).
+async function unlink(userId, provider) {
+  return db.deleteForUserProvider(userId, provider)
+}
+
+module.exports = { findByProviderSubject, listForUser, link, unlink }