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.

tests/test_classes.py

@@ -11,8 +11,12 @@ from conftest import make_participants, make_timedelta_matrix
 
 class TestParticipant:
     def test_uuid_is_unique(self):
-        a = Participant("A", "addr", "", 5, "")
-        b = Participant("A", "addr", "", 5, "")
+        a = Participant(
+            name="A", address="addr", phone="", kitchen_size=5, allergies=""
+        )
+        b = Participant(
+            name="A", address="addr", phone="", kitchen_size=5, allergies=""
+        )
         assert a.uuid != b.uuid
 
     @pytest.mark.parametrize(
@@ -20,7 +24,9 @@ class TestParticipant:
         [(10, 0), (7, 9), (0, 30), (5, 15)],
     )
     def test_penalty_scales_with_kitchen_size(self, kitchen_size, minutes):
-        p = Participant("A", "addr", "", kitchen_size, "")
+        p = Participant(
+            name="A", address="addr", phone="", kitchen_size=kitchen_size, allergies=""
+        )
         assert p.get_penalty() == dt.timedelta(minutes=minutes)
 
     def test_after_party_time_combines_penalty_and_travel(self):
@@ -32,7 +38,13 @@ class TestParticipant:
         assert result == dt.timedelta(minutes=9) + dt.timedelta(seconds=600)
 
     def test_dict_roundtrip_fields(self):
-        p = Participant("Alice", "addr", "555", 8, "peanuts")
+        p = Participant(
+            name="Alice",
+            address="addr",
+            phone="555",
+            kitchen_size=8,
+            allergies="peanuts",
+        )
         d = p.dict()
         assert d == {
             "uuid": p.uuid,