Rewrite

[lmaotrigine]

• Drop Rocket and Tera, switch to Axum.
• Use procedural macros to embed HTML into the executable.
• Fix weird OnceCell init errors (probably related to upstream Undiagnosed rare bug will set heartbeatStats.LongestMissingBeat to a much higher number than is accurate 5HT2B/heartbeat#29)
• Use a websocket to send stats (currently it just sends an update every second. This can be made more efficient using broadcast channels, but that's a problem for future me). Addresses upstream Use streaming connection for live updates 5HT2B/heartbeat#23
• Update MSRV and dependencies.