Add 24-hour TTL to Eurostar price cache

Cache reads now accept an optional ttl (seconds). get_cached checks the
file mtime and returns None if the entry is older than the TTL, triggering
a fresh fetch. Eurostar prices use a 24-hour TTL; timetable caches remain
indefinite (date-scoped keys become irrelevant once the date passes).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_cache.py

@@ -0,0 +1,42 @@
+import os
+import time
+import pytest
+from cache import get_cached, set_cached
+
+
+@pytest.fixture
+def tmp_cache(tmp_path, monkeypatch):
+    import cache as cache_module
+    monkeypatch.setattr(cache_module, 'CACHE_DIR', str(tmp_path))
+    return tmp_path
+
+
+def test_get_cached_returns_none_for_missing_key(tmp_cache):
+    assert get_cached('no_such_key') is None
+
+
+def test_set_and_get_cached_roundtrip(tmp_cache):
+    set_cached('my_key', {'a': 1})
+    assert get_cached('my_key') == {'a': 1}
+
+
+def test_get_cached_no_ttl_never_expires(tmp_cache):
+    set_cached('k', [1, 2, 3])
+    # Backdate the file by 2 days
+    path = tmp_cache / 'k.json'
+    old = time.time() - 2 * 86400
+    os.utime(path, (old, old))
+    assert get_cached('k') == [1, 2, 3]
+
+
+def test_get_cached_within_ttl(tmp_cache):
+    set_cached('k', 'fresh')
+    assert get_cached('k', ttl=3600) == 'fresh'
+
+
+def test_get_cached_expired_returns_none(tmp_cache):
+    set_cached('k', 'stale')
+    path = tmp_cache / 'k.json'
+    old = time.time() - 25 * 3600  # 25 hours ago
+    os.utime(path, (old, old))
+    assert get_cached('k', ttl=24 * 3600) is None