Completes the parked MIDI feature (midimon.py + midistream.py were done/tested in
270a01d6; this wires them into the always-on Bridge). The "do better than a raw
aseqdump in a terminal" monitor now lives in the web dashboard.

server.py: a module-global MidiStream fan-out (lazy — opens aseqdump only while a
tab is watching, closes on the last unsubscribe). Two GET routes:
  /api/midi/ports  → list ALSA seq ports
  /api/midi/stream → Server-Sent Events of parsed events, ?port=addr to pick one.
ThreadingHTTPServer gives each SSE connection its own thread, so blocking on the
subscriber queue is fine; a ': ping' every 15 s holds the pipe open, and
BrokenPipe/ConnectionReset on tab-close is swallowed and triggers unsubscribe.

ui/index.html: a MIDI panel — port picker + connect/disconnect (EventSource) +
rescan, and a live log. Rows are colour-coded by event (note-on green, note-off
faint, control-change amber, pitch-bend magenta), show source/event/channel/
note-name and a velocity bar (val/127), newest on top, capped at 200. Never hue
alone: the event word carries the meaning, colour reinforces (DESIGN principle 4).

Verified end to end: systemd --user restart, /api/midi/ports lists the seq ports,
/api/midi/stream emits ': connected' then live events, and the UI connects (green
"live") and renders rows — no console errors. With no hardware controller attached
the feed shows system/subscription events; note/CC rows appear once a controller
port is selected and played.