The /terminals modal and the status-bar count need a server source for the
session's persistent terminals (background dev servers, etc.) and a way to
force-close one. Add REST endpoints over the existing TerminalManager:

- GET /sessions/{id}/terminals → terminal_manager.list(session_id) →
  {session_id, terminals: [summary,...]}. Access-checked (navi.sessions.read_all).
- POST /sessions/{id}/terminals/{name}/close → terminal_manager.close(...)
  → {session_id, terminal_name, closed: bool}. Name is URL-encoded by the
  client (terminal names may contain spaces).
- clients/terminal/api.py (async): list_terminals(session_id),
  close_terminal(session_id, name) — for the TUI modal + status-bar seed.

Tests: list empty / list returns active / close routes to manager / 404 on
unknown session (fake TerminalManager injected into the container, mirroring the
kv_store injection pattern). Full suite: 972 passed, 1 skipped.

Plan: docs/terminal_tool_plan.md — Etap 2 of 5.