[Enhancement] Web config panel for roster / schedule / message templates

[perrycan]

Currently the group name, roster, name map, schedule times and message templates all live in config.py and require editing source.

A lightweight web panel would be nice (FastAPI is already running):

• Edit the member list and exclusion list online;
• Adjust the three schedule times and message templates;
• Apply without restart (consistent with rule hot-reload).

Good first task.