feat(auth): SA4 — server-side session revocation via per-user session epoch

Signed client-side session cookies had no server-side kill switch: logout + password
reset couldn't evict a stolen/copied cookie (valid until expiry). Add User.session_epoch
(migration 0053), record it in the cookie at every login, and reject in current_user any
cookie whose recorded epoch is behind the account's current one.
- Bump the epoch on: password reset (kills ALL sessions — a reset is a compromise response),
  password change + a new 'Log out other sessions' action (both re-stamp the CURRENT cookie
  so the caller stays signed in, evicting only the others).
- Per-account epoch map in the session so one account's revocation doesn't evict the other
  signed-in accounts in the same browser wallet.
- Missing epoch (pre-SA4 cookie) is treated as 0, so the first bump revokes grandfathered
  sessions too.
- New POST /auth/logout-others + a Settings → Account 'Active sessions' button (trilingual).

frontend/src/lib/api.ts

@@ -1394,6 +1394,8 @@ export const api = {
   // Confirm an email-verification token the SPA read from the URL fragment (SB3).
   verifyEmail: (token: string): Promise<{ ok: boolean }> =>
     req("/auth/verify", { method: "POST", body: JSON.stringify({ token }) }, { quiet: true }),
+  // SA4: sign every OTHER session for this account out (keeps the current one).
+  logoutOthers: (): Promise<{ ok: boolean }> => req("/auth/logout-others", { method: "POST" }),
   // Set or change the signed-in account's password (errors shown inline via quiet).
   setPassword: (password: string, currentPassword?: string): Promise<{ ok: boolean }> =>
     req(