fix: strip checkin_instructions from public room dicts (privacy leak)

The AUTH_NONE RPC room endpoints (chatelet_room_list/_get) stripped wallet
but NOT checkin_instructions — which was added in #5 after this code, so the
operator's private access details (address, gate code) were leaking to any
guest. Centralize a public_room_dict(room) helper (strips wallet +
checkin_instructions) and route both doors through it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019VUQCfdqiLSsFS2jcGnaFD

models.py

@@ -17,6 +17,7 @@ Design notes carried into the field definitions:
   arbiter of "is this range open" — Nostr events are requests, not locks.
 """
 
+import json
 from datetime import datetime, timezone
 from enum import Enum
 
@@ -194,6 +195,17 @@ class Block(BaseModel):
 # ---------------------------------------------------------------------------
 
 
+def public_room_dict(room: Room) -> dict:
+    """A Room as public JSON for guests — strips operator-private fields: the
+    wallet id, and the check-in instructions (address/gate code, delivered only
+    in the encrypted post-payment DM). Shared by the HTTP and Nostr-RPC guest
+    doors so neither can leak them."""
+    d = json.loads(room.json())
+    d.pop("wallet", None)
+    d.pop("checkin_instructions", None)
+    return d
+
+
 class AvailabilityQuery(BaseModel):
     room_id: str
     check_in: str                        # YYYY-MM-DD inclusive