svc: api: Get URLs file path from env var

The `HUDCTRL_URLS_FILE` environment variable can be used to specify the
location to the `urls.json` file.

svc/src/hudctrl/api.py

@@ -1,9 +1,11 @@
 import io
 import logging
+import os
+from pathlib import Path
 from typing import Optional
 
-import fastapi
 from PIL import Image
+import fastapi
 
 from .hud import HUDService, NoMonitorConfig
 from .xrandr import MonitorConfig
@@ -15,11 +17,16 @@ log = logging.getLogger(__name__)
 logging.basicConfig(level=logging.DEBUG)
 
 
+HUDCTRL_URLS_FILE = os.environ.get('HUDCTRL_URLS_FILE')
+
+
 app = fastapi.FastAPI(
     docs_url='/api-doc/',
 )
 
 svc = HUDService()
+if HUDCTRL_URLS_FILE:
+    svc.urls_file = Path(HUDCTRL_URLS_FILE)
 
 
 class PNGImageResponse(fastapi.Response):