chore: scaffold chatelet LNbits extension

Nostr-native room rentals (Airbnb-style) for the castle. Establishes the
extension manifest, license, README with the design overview, and static
+ template placeholders. No booking logic yet — subsequent commits build
the model, schema, arbiter, Nostr layer, API, tasks, and wiring bottom-up.

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

README.md

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+# Chatelet
+
+Nostr-native room rentals for LNbits — an "Airbnb for the castle". List
+rooms, take booking requests, arbitrate availability, and settle stays over
+Lightning, with Nostr as the interop layer.
+
+> **Status:** design sketch + scaffold. Data model, migrations, availability
+> arbiter, REST surface, and Nostr event model are in place; relay plumbing,
+> FX/invoice wiring, and the guest UI are stubbed with `TODO(...)` markers.
+
+## Why not just reuse an existing NIP?
+
+No single NIP models a rental-booking flow. Chatelet **composes** existing
+NIPs and only invents an event kind where nothing fits:
+
+| Concern | Mechanism |
+|---|---|
+| Room listing | **NIP-99** classified listing (`kind:30402`) |
+| Guest's reservation record | **NIP-78** app data (`kind:30078`), NIP-44 encrypted |
+| Public availability calendar | **NIP-52** (`kind:31923/31924`), no guest PII |
+| Private request → quote → confirm | **NIP-17/59** giftwrapped DMs |
+| Live "is it free?" query | aiolabs band **`kind:22000/22001`** (ephemeral) |
+| Payment / deposit | LNbits Lightning invoice |
+
+Full rationale in [`docs/adr-0001-nostr-event-model.md`](docs/adr-0001-nostr-event-model.md).
+
+## The one invariant that matters
+
+**LNbits is the booking authority; Nostr events are requests, not locks.**
+Availability is *derived* from the DB (`bookings` + `blocks`), never stored
+as a positive fact, and a date range is only truly locked when a `held`
+booking row is written. Payment — not any Nostr event — promotes a booking
+to `confirmed`.
+
+## Docs
+
+- [`docs/data-model.md`](docs/data-model.md) — tables, entities, lifecycle
+- [`docs/event-flow.md`](docs/event-flow.md) — who publishes what, when (with sequence diagram)
+- [`docs/adr-0001-nostr-event-model.md`](docs/adr-0001-nostr-event-model.md) — why these kinds
+
+## Layout
+
+```
+config.json        LNbits extension manifest
+models.py          data model (pydantic)
+migrations.py      schema (ext_chatelet DB)
+crud.py            persistence + the availability arbiter
+views_api.py       REST surface (shares the booking flow with Nostr)
+views.py           operator admin page route
+tasks.py           invoice listener (settle) + hold-expiry sweep
+nostr/             event kinds, builders, sign/publish/subscribe service
+docs/              design docs
+```