fix(models): restore check_in/check_out on AvailabilityQuery

v0.4.0's merge of public_booking_dict landed inside AvailabilityQuery and
left the two date fields orphaned after the helper's return, so
POST /api/v1/availability raised AttributeError on q.check_in (500) on
every install of that version. The RPC door reads the raw body and was
unaffected, which is why nothing noticed.

Put the helper next to public_room_dict and add a regression test that
constructs the query model and drives the endpoint through the
services layer.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

models.py

@@ -206,8 +206,6 @@ def public_room_dict(room: Room) -> dict:
     return d
 
 
-class AvailabilityQuery(BaseModel):
-    room_id: str
 def public_booking_dict(booking: "Booking") -> dict:
     """A Booking as public JSON for the guest who holds its id — lifecycle +
     money + dates only. Strips the guest's own identity/contact (so the id
@@ -227,8 +225,10 @@ def public_booking_dict(booking: "Booking") -> dict:
     return d
 
 
-    check_in: str                        # YYYY-MM-DD inclusive
-    check_out: str                       # YYYY-MM-DD exclusive
+class AvailabilityQuery(BaseModel):
+    room_id: str
+    check_in: str  # YYYY-MM-DD inclusive
+    check_out: str  # YYYY-MM-DD exclusive
 
 
 class AvailabilityResult(BaseModel):

tests/test_availability_endpoint.py

@@ -0,0 +1,56 @@
+"""Guest availability endpoint. Regression guard for v0.4.0, where a bad merge
+left AvailabilityQuery with only `room_id` and POST /availability 500'd on
+`q.check_in` — the RPC door reads the raw body, so only HTTP was broken and no
+test exercised it."""
+
+import asyncio
+
+import pytest
+from fastapi import HTTPException
+from pydantic import ValidationError
+
+from .. import services, views_api
+from ..models import AvailabilityQuery, AvailabilityResult
+
+
+def test_availability_query_requires_both_dates():
+    q = AvailabilityQuery(room_id="r", check_in="2026-10-01", check_out="2026-10-03")
+    assert (q.room_id, q.check_in, q.check_out) == ("r", "2026-10-01", "2026-10-03")
+    assert set(AvailabilityQuery.__fields__) == {"room_id", "check_in", "check_out"}
+    with pytest.raises(ValidationError):
+        AvailabilityQuery(room_id="r")  # type: ignore[call-arg]
+
+
+def test_availability_endpoint_forwards_all_three_fields(monkeypatch):
+    seen: list[tuple] = []
+
+    async def fake(room_id, check_in, check_out):
+        seen.append((room_id, check_in, check_out))
+        return AvailabilityResult(
+            room_id=room_id,
+            check_in=check_in,
+            check_out=check_out,
+            available=True,
+            nights=2,
+        )
+
+    monkeypatch.setattr(services, "get_availability", fake)
+    q = AvailabilityQuery(room_id="r", check_in="2026-10-01", check_out="2026-10-03")
+    out = asyncio.run(views_api.api_check_availability(q))
+    assert seen == [("r", "2026-10-01", "2026-10-03")]
+    assert out.available and out.nights == 2
+
+
+@pytest.mark.parametrize(
+    ("exc", "status"),
+    [(services.NotFound("Room not found"), 404), (ValueError("bad dates"), 400)],
+)
+def test_availability_endpoint_maps_service_errors(monkeypatch, exc, status):
+    async def fake(*_):
+        raise exc
+
+    monkeypatch.setattr(services, "get_availability", fake)
+    q = AvailabilityQuery(room_id="r", check_in="2026-10-03", check_out="2026-10-01")
+    with pytest.raises(HTTPException) as e:
+        asyncio.run(views_api.api_check_availability(q))
+    assert e.value.status_code == status