Implement web session/token revocation (#30)

Web sessions were stateless JWTs with no server-side store: the revocation
hooks in session.service were stubs that only logged. As a result web logout
was client-side only (a copied cookie stayed valid until natural JWT expiry)
and a password change never invalidated existing sessions. The mobile bearer
flow already had revocable, DB-stored tokens; this brings the web/cookie flow
to parity.

Two-layer revocation, both enforced in requireAuth (which already loads the
fresh user row each request):

- Per-session denylist: new `revoked_sessions` table keyed on the JWT `jti`
  (already minted per session). A single logout adds this session's jti;
  rows self-expire at the token's own exp and are pruned on boot. New model
  `revokedSessions` mirrors the `mobileSessions` db/model split.
- Per-user cutoff: new `users.tokens_valid_after` column. A password change
  (and the new `invalidateSessions` helper) bumps it to NOW(); any token whose
  iat is at or before the cutoff is rejected. The comparison is inclusive so a
  token minted in the same wall-clock second as the change is still revoked.

Wiring:
- session.service: revokeSession / invalidateSession / invalidateAllUserSessions
  now delegate to the stores; sessions carry `expiresAt` (JWT exp) so logout can
  set a self-pruning denylist row.
- /logout gains best-effort attachSession so the controller can revoke this
  session's jti and log auth.logout; stays a no-op for anonymous callers.
- users.model.update bumps the cutoff whenever the password hash is rotated.
- schema.sql: revoked_sessions table + tokens_valid_after column, added to the
  CREATE and to the idempotent migration block (ensureSchema on boot).

Verified end-to-end against the local dev DB: a captured cookie is rejected
after logout, and an existing session is rejected after a password change while
re-login with the new password succeeds. Full server test suite green (96).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019rao86n5cXpwAyjdBFEshV

server/src/model/users/users.db.js

@@ -54,6 +54,13 @@ async function touchLastLogin(id) {
   return query('UPDATE users SET last_login_at = NOW() WHERE id = ?', [id])
 }
 
+// Move the "tokens valid after" cutoff to now, invalidating every session token
+// issued before this instant (password change / log out everywhere). requireAuth
+// compares each session's issued-at against this column.
+async function bumpTokensValidAfter(id) {
+  return query('UPDATE users SET tokens_valid_after = NOW() WHERE id = ?', [id])
+}
+
 // Store a (not-yet-enabled) TOTP secret for a user. Enabling is a separate step
 // so a secret is never trusted until the user has confirmed one code.
 async function setTotpSecret(id, secret) {
@@ -78,6 +85,7 @@ module.exports = {
   countUsers,
   countAdmins,
   touchLastLogin,
+  bumpTokensValidAfter,
   setTotpSecret,
   enableTotp,
   disableTotp,