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/authProviders/authProviders.db.js

@@ -0,0 +1,36 @@
+const { query } = require('../../utils/db')
+
+const COLS =
+  'id, kind, name, enabled, client_id, client_secret_enc, authorize_url, token_url, userinfo_url, scopes, priority, created_at, updated_at'
+
+async function list() {
+  return query(`SELECT ${COLS} FROM auth_providers ORDER BY priority ASC, id ASC`)
+}
+
+async function get(id) {
+  const rows = await query(`SELECT ${COLS} FROM auth_providers WHERE id = ? LIMIT 1`, [id])
+  return rows[0] || null
+}
+
+// Upsert a provider row. `fields` are column values already prepared by the model
+// (secret pre-encrypted). Only the provided columns are written/updated.
+async function upsert(id, fields) {
+  const cols = Object.keys(fields)
+  const vals = cols.map((c) => fields[c])
+  const insertCols = ['id', ...cols].map((c) => `\`${c}\``).join(', ')
+  const placeholders = ['?', ...cols.map(() => '?')].join(', ')
+  const updates = cols.map((c) => `\`${c}\` = VALUES(\`${c}\`)`).join(', ')
+  await query(
+    `INSERT INTO auth_providers (${insertCols}) VALUES (${placeholders})
+     ON DUPLICATE KEY UPDATE ${updates}`,
+    [id, ...vals],
+  )
+  return get(id)
+}
+
+async function remove(id) {
+  const res = await query('DELETE FROM auth_providers WHERE id = ?', [id])
+  return Number(res.affectedRows || 0)
+}
+
+module.exports = { list, get, upsert, remove }

server/src/model/authProviders/authProviders.model.js

@@ -0,0 +1,50 @@
+// Auth-provider config store. Thin logic layer over authProviders.db, mirroring
+// the users model split. Owns encryption of the client secret at the boundary so
+// the DB layer only ever sees ciphertext and callers only ever see the decrypted
+// secret when they explicitly ask (getWithSecret) — the plain list/get paths
+// never surface it.
+
+const db = require('./authProviders.db')
+const secretBox = require('../../utils/secretBox')
+
+// All configured provider rows (secret column left as ciphertext; callers that
+// need the secret use getWithSecret).
+async function list() {
+  return db.list()
+}
+
+async function get(id) {
+  return db.get(id)
+}
+
+// Provider row with the client secret decrypted (server-side only — used by the
+// registry at token-exchange time). Returns null if the provider does not exist.
+async function getWithSecret(id) {
+  const row = await db.get(id)
+  if (!row) return null
+  return { ...row, client_secret: row.client_secret_enc ? secretBox.decrypt(row.client_secret_enc) : null }
+}
+
+// Create/update a provider. `secret` (raw) is encrypted here; pass secret ===
+// undefined to leave an existing secret untouched, or '' to keep it unchanged as
+// well (blank means "no change" from the admin UI). Returns the stored row.
+async function save(id, { kind, name, enabled, clientId, secret, authorizeUrl, tokenUrl, userinfoUrl, scopes, priority }) {
+  const fields = {}
+  if (kind !== undefined) fields.kind = kind
+  if (name !== undefined) fields.name = name
+  if (enabled !== undefined) fields.enabled = enabled ? 1 : 0
+  if (clientId !== undefined) fields.client_id = clientId
+  if (secret) fields.client_secret_enc = secretBox.encrypt(secret) // only when a new secret is given
+  if (authorizeUrl !== undefined) fields.authorize_url = authorizeUrl
+  if (tokenUrl !== undefined) fields.token_url = tokenUrl
+  if (userinfoUrl !== undefined) fields.userinfo_url = userinfoUrl
+  if (scopes !== undefined) fields.scopes = scopes
+  if (priority !== undefined) fields.priority = priority
+  return db.upsert(id, fields)
+}
+
+async function remove(id) {
+  return db.remove(id)
+}
+
+module.exports = { list, get, getWithSecret, save, remove }