Add config system with TFL_DATA_DIR and CACHE_DIR

config/default.py holds defaults using ~/lib/data/tfl (expanduser, so safe
to commit). app.py loads it then overlays config/local.py if present, pushing
paths into cache and circle_line modules. config/local.py is gitignored for
machine-specific absolute paths (e.g. on the server where www-data runs).

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

app.py

@@ -3,6 +3,7 @@ Combine GWR Bristol→Paddington trains with Eurostar St Pancras→destination t
 """
 from flask import Flask, render_template, redirect, url_for, request
 from datetime import date, timedelta
+import os
 
 from cache import get_cached, set_cached
 import scraper.eurostar as eurostar_scraper
@@ -22,7 +23,16 @@ RTT_BRISTOL_URL = (
     "?stp=WVS&show=pax-calls&order=wtt"
 )
 
-app = Flask(__name__)
+app = Flask(__name__, instance_relative_config=False)
+app.config.from_object('config.default')
+_local = os.path.join(os.path.dirname(__file__), 'config', 'local.py')
+if os.path.exists(_local):
+    app.config.from_pyfile(_local)
+
+import cache
+import circle_line
+cache.CACHE_DIR = app.config['CACHE_DIR']
+circle_line._TXC_XML = app.config['CIRCLE_LINE_XML']
 
 DESTINATIONS = {
     'paris':      'Paris Gare du Nord',