Add TODO.md and reframe CLAUDE.md around REST-push philosophy

Kao is a display, not a monitor — external systems push events via REST
rather than Kao polling things itself. Update CLAUDE.md to reflect this:
- New Design Philosophy section making the REST-first approach explicit
- Architecture diagram updated to show external systems as the push source
- Detectors section demoted to "Legacy Detectors" with a note to prefer push
- TODO.md added with planned REST API improvements (SSE, notify queue,
  sticky notifications, named presets, batch notify, /history endpoint)
  and display improvements (brightness curve, scrolling ticker)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TODO.md

@@ -0,0 +1,26 @@
+# Kao — TODO
+
+Feature ideas for future work, roughly in priority order.
+
+## REST API improvements
+
+- **`/status` SSE stream** — replace frontend polling with a Server-Sent Events
+  endpoint so the display reacts instantly and the 2s polling overhead disappears
+- **Notification queue** — buffer rapid `/notify` calls and auto-advance through
+  them instead of clobbering; important when HA fires several events at once
+- **Sticky notifications** — a `sticky: true` flag on `/notify` to keep a
+  notification visible until explicitly cleared via `/clear`, rather than TTL-expiring
+- **Named presets** — define reusable notification profiles in `config.json`
+  (e.g. `"doorbell"` → specific emote/sound/color/duration) so callers can
+  POST `{"preset": "doorbell"}` without repeating fields every time
+- **Batch `/notify`** — accept an array in a single POST so multiple
+  notifications can be queued atomically
+- **`/history` endpoint** — a ring buffer of the last N state changes/events
+  received, for auditing what fired overnight without tailing logs
+
+## Display / frontend
+
+- **Brightness curve** — dim gradually after dark rather than hard-sleeping;
+  reduces OLED burn-in without losing glanceability
+- **Scrolling ticker** — when multiple events are active, cycle through their
+  messages rather than only showing the top one