Combine --watch functionality with --server

[victor-hucklenbroich]

RESOLVES #94

Add a persistent BLE notification subscription to the HTTP server, gated
on a new watch: bool config option. When enabled, the server mirrors
desk state changes to stdout (matching the CLI --watch behaviour) and
exposes a new /events websocket endpoint that streams JSON updates of
the form {"height": , "speed": <mm/s>} to connected clients.

To support multiple consumers of the same notification stream, the Desk
class gains an in-process pub/sub layer (subscribe / unsubscribe ->
start_watching / stop_watching) with a cached latest_height and
latest_speed. The existing watch_height_speed CLI entrypoint is
reimplemented on top of this layer with no change in behaviour.

The TCP server is unchanged and remains request/response only.

Adds corresponding new flag (watch: True / False) to config.yaml. Defaults to false, so existing configs are unaffected until they opt in.

Re-establish watch subscription after reconnect

While working on the server watch changes I found a pre-existing gap in the reconnect handling. disconnect_callback transparently reconnects the BLE client, but the notification subscription dies with the old connection while desk._watching stays True. After a reconnect the subscription is never rebuilt and the if self._watching: return guard in start_watching would prevent it anyway so the height/speed event stream goes silently dead.

This affects both the new --server --watch path and the existing CLI --watch. It isn't introduced by the server watch changes, but a long-running server will eventually hit it, so it's worth fixing here.

The fix is small: Desk.mark_disconnected() clears the watching flag on disconnect and reports whether watching was active, and connect re-calls start_watching on the new connection when it was. Existing subscribers are preserved, so clients keep receiving events seamlessly across a reconnect.

Functionality (new and existing) is manually tested on my local macOS machine with my Ikea Idasen