Participant and Group are now pydantic BaseModels. Group.hosts can form
cycles between groups, so it's kept as a private, non-persisted live list
(set via set_hosts()/add_host()) backed by a serializable host_uuids field,
re-linked via resolve_hosts() after a reload.
A new Plan model (src/tatami/plan.py) bundles groups, the after-party group,
and the event config (course_times, organizer_contacts, info_text,
spreadsheet_id) and supports save()/load() to/from JSON. tatami_masterplan's
__main__ now saves to masterplan.json (PLAN_FILE env var to override) on
first run and loads it on later runs instead of recomputing, so the plan can
be hand-edited (move a member between groups, change a course, fill in
spreadsheet_id) and picked up on rerun without hitting the Routes API again.
spreadsheet_id moves out of .env (GOOGLE_SHEETS_SPREADSHEET_ID) onto the plan
itself, since it's part of the plan rather than a secret.
Lets organizers automatically populate a shared Google Sheet (the same kind
they previously built by hand) with an Overview tab and one tab per group,
instead of printing dicts. Group tabs are named after all their members
(not a single host) and show each group's route, course times, and guests
with allergies. The Overview tab also gets configurable Meal Times, Support
Contacts, and Info sections, passed through as plain data from
tatami_masterplan.py. Export is fully opt-in via GOOGLE_SHEETS_CREDENTIALS_FILE
and GOOGLE_SHEETS_SPREADSHEET_ID; without them, behavior is unchanged.
lars