Add ACE-Step 1.5 wizard soundtrack prompts

[Copilot]

New docs/SOUNDTRACK_PROMPTS.md — music generation prompt reference for ACE-Step 1.5, mirroring the pattern of ART_AND_CHARACTERS.md for audio instead of visuals.

• Style guide with base style suffix, retro variant, and production notes (looping, duration, tempo ranges)
• 13 instrument-only prompts covering all game scenes: main theme, exploration, battle, library, potion lab, broom racing, dungeon, shop, victory fanfare, defeat, villain, fellowship hall, night ambience
• 5 vocal prompts with full lyrics using [verse]/[chorus]/[bridge]/[outro] structure: academy anthem, spell duel ballad, tavern song, frost sage lullaby, storm caller anthem
• Usage notes for generation workflow and frontend integration (frontend/public/audio/)

Example instrumental prompt:

Fantasy ambient, whimsical, 78 BPM, plucked harp, soft flute melody, gentle pizzicato strings,
celesta, light woodwind countermelody, airy pads, seamless loop, warm room reverb, no vocals.