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>

cache.py

@@ -1,5 +1,6 @@
 import json
 import os
+import time
 
 CACHE_DIR = os.path.join(os.path.dirname(__file__), 'cache')
 
@@ -9,10 +10,13 @@ def _cache_path(key: str) -> str:
     return os.path.join(CACHE_DIR, f"{safe_key}.json")
 
 
-def get_cached(key: str):
+def get_cached(key: str, ttl: int | None = None):
+    """Return cached data, or None if missing or older than ttl seconds."""
     path = _cache_path(key)
     if not os.path.exists(path):
         return None
+    if ttl is not None and time.time() - os.path.getmtime(path) > ttl:
+        return None
     with open(path) as f:
         return json.load(f)