Add per-node scene music volume control

Author a music track and volume (0-100%) per story node; the runtime
scales it to 0-1 and applies it without restarting the track when only
the level changes. Adds migration 024, threads music_volume through the
repository/runtime, and gives the node inspector a volume slider.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

migrations/024_story_node_music_volume.sql

@@ -0,0 +1,4 @@
+-- Per-node scene-music volume (0-100%). Applied whenever a node sets a track;
+-- selecting the same track on a later node with a different volume just adjusts
+-- the level without restarting the music.
+ALTER TABLE osint.story_nodes ADD COLUMN music_volume SMALLINT NOT NULL DEFAULT 100 CHECK (music_volume BETWEEN 0 AND 100);