feat(music): let the dashboard add songs, starting a session if needed

New AddToQueue appends to an already-active session without touching
voice (the dashboard can't join a channel on its own), and add_song's
dispatch case now branches: a voice_channel_id in the payload means
"start fresh", so it goes through EnqueueAndPlay (join + play) instead.
Query normalization (plain text -> YouTube search, stripping
autoplay/radio params off pasted YouTube URLs) moves out of /play's
handler into a shared NormalizeQuery, both entry points need it, not
just one.

Starting playback from the dashboard also posts the now-playing embed
into the voice channel's own built-in text chat, matching what /play
already does, instead of leaving it with nowhere to show up.

schema.sql

@@ -170,7 +170,7 @@ CREATE TABLE IF NOT EXISTS music_queue (
 CREATE TABLE IF NOT EXISTS music_commands (
     id BIGSERIAL PRIMARY KEY,
     guild_id BIGINT NOT NULL,
-    command_type TEXT NOT NULL CHECK (command_type IN ('skip', 'pause', 'resume', 'set_volume', 'remove_track', 'reorder', 'repeat_song', 'repeat_queue')),
+    command_type TEXT NOT NULL CHECK (command_type IN ('skip', 'pause', 'resume', 'set_volume', 'remove_track', 'reorder', 'repeat_song', 'repeat_queue', 'add_song')),
     payload JSONB,
     requested_by BIGINT NOT NULL,
     created_at TIMESTAMPTZ DEFAULT NOW(),