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_e2e_sheets.py

@@ -71,57 +71,69 @@ def _make_mock_groups() -> tuple[list[Group], Group]:
     """3 groups of 3 fictional participants with addresses around Karlsruhe."""
     hosts = [
         Participant(
-            "Anna Wagner", "Kaiserstraße 12, 76131 Karlsruhe", "0721-1000001", 9, "none"
+            name="Anna Wagner",
+            address="Kaiserstraße 12, 76131 Karlsruhe",
+            phone="0721-1000001",
+            kitchen_size=9,
+            allergies="none",
         ),
         Participant(
-            "Jonas Becker",
-            "Waldstraße 5, 76133 Karlsruhe",
-            "0721-1000002",
-            7,
-            "lactose",
+            name="Jonas Becker",
+            address="Waldstraße 5, 76133 Karlsruhe",
+            phone="0721-1000002",
+            kitchen_size=7,
+            allergies="lactose",
         ),
         Participant(
-            "Mira Hofmann", "Yorckstraße 22, 76185 Karlsruhe", "0721-1000003", 8, "none"
+            name="Mira Hofmann",
+            address="Yorckstraße 22, 76185 Karlsruhe",
+            phone="0721-1000003",
+            kitchen_size=8,
+            allergies="none",
         ),
     ]
     semi_hosts = [
         Participant(
-            "Lukas Schreiber",
-            "Sophienstraße 40, 76135 Karlsruhe",
-            "0721-1000004",
-            5,
-            "nuts",
+            name="Lukas Schreiber",
+            address="Sophienstraße 40, 76135 Karlsruhe",
+            phone="0721-1000004",
+            kitchen_size=5,
+            allergies="nuts",
         ),
         Participant(
-            "Sophie Lindner",
-            "Beiertheimer Allee 18, 76137 Karlsruhe",
-            "0721-1000005",
-            6,
-            "none",
+            name="Sophie Lindner",
+            address="Beiertheimer Allee 18, 76137 Karlsruhe",
+            phone="0721-1000005",
+            kitchen_size=6,
+            allergies="none",
         ),
         Participant(
-            "Tom Vogel",
-            "Durlacher Allee 75, 76131 Karlsruhe",
-            "0721-1000006",
-            4,
-            "none",
+            name="Tom Vogel",
+            address="Durlacher Allee 75, 76131 Karlsruhe",
+            phone="0721-1000006",
+            kitchen_size=4,
+            allergies="none",
         ),
         Participant(
-            "Lea Brandt",
-            "Moltkestraße 30, 76133 Karlsruhe",
-            "0721-1000007",
-            3,
-            "gluten",
+            name="Lea Brandt",
+            address="Moltkestraße 30, 76133 Karlsruhe",
+            phone="0721-1000007",
+            kitchen_size=3,
+            allergies="gluten",
         ),
         Participant(
-            "Felix Krause", "Adlerstraße 14, 76133 Karlsruhe", "0721-1000008", 8, "none"
+            name="Felix Krause",
+            address="Adlerstraße 14, 76133 Karlsruhe",
+            phone="0721-1000008",
+            kitchen_size=8,
+            allergies="none",
         ),
         Participant(
-            "Nora Fink",
-            "Rüppurrer Straße 60, 76137 Karlsruhe",
-            "0721-1000009",
-            5,
-            "none",
+            name="Nora Fink",
+            address="Rüppurrer Straße 60, 76137 Karlsruhe",
+            phone="0721-1000009",
+            kitchen_size=5,
+            allergies="none",
         ),
     ]