multi-user auth (CLI-created accounts), per-user history, test suite

- server/users.py replaces credentials.py: users.json (0600), PBKDF2,
  one-time migration of the legacy single password to user admin
  (history.json -> history/admin.json)
- accounts are CLI-managed only: python -m server.users add|rm|list
  (no self-signup on an internet-exposed app); /signup route removed,
  login page back to username + password
- per-user 30-day signed session cookie (payload carries the
  username), per-user lockout, GET /api/me, POST /account/delete
- per-user watch history: server/data/history/<user>.json; ANIWEB_DATA
  env override for the data dir
- header chip: logged-in user, logout, delete account
- tests/: throwaway-server suite (port 8399, temp data dir) — 9 HTTP
  tests + 2 Playwright browser tests (login UI, real video playback
  behind auth); runs via python tests/run.py

static/style.css

@@ -6,6 +6,10 @@ header { display: flex; gap: 1rem; align-items: center; padding: 0.8rem 1.2rem;
   border-bottom: 1px solid var(--bd); position: sticky; top: 0; background: var(--bg); z-index: 1; }
 .logo { font-weight: 700; font-size: 1.2rem; color: var(--acc); }
 header form { flex: 1; max-width: 420px; }
+button.tiny { padding: .2rem .55rem; font-size: .8rem; border-radius: 6px;
+  border: 1px solid var(--bd); background: var(--card); color: var(--fg); cursor: pointer; }
+button.tiny.danger { border-color: #7f1d1d; color: #f87171; }
+header #me { margin-left: auto; }
 #q { width: 100%; padding: 0.5rem 0.8rem; border-radius: 8px; border: 1px solid var(--bd);
   background: var(--card); color: var(--fg); }
 main { padding: 1.2rem; max-width: 1100px; margin: 0 auto; }