Move Participant/Group to pydantic and add a saveable/editable Plan model

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.

pyproject.toml

@@ -9,6 +9,7 @@ dependencies = [
     "numpy>=2.2.4",
     "pandas>=2.2.3",
     "pandas-stubs>=2.2.3.250308",
+    "pydantic>=2.13.4",
     "python-dotenv>=1.2.2",
     "requests>=2.32.3",
     "tqdm>=4.67.1",