feat(minimax): add image/audio/video/music model entries (#77)

Extend modality enum to support video and music, then register the
non-text MiniMax models that were already declared in
media_capabilities but had no concrete entries:

- image-01 ($0.0035/image)
- speech-2.8/2.6 hd & turbo ($60-$100 per 1M chars)
- Hailuo 2.3 Fast / 2.3 / 02 video models ($0.10-$0.56 per video)
- music-2.6, lyrics_generation

Per-call pricing is documented in inline comments since the schema's
token-based cost fields don't naturally fit per-call billing.

schema.toml and scripts/validate.py both updated; the change is
additive (existing modality values remain valid).

scripts/validate.py

@@ -34,7 +34,7 @@ except ImportError:
         sys.exit(1)
 
 VALID_TIERS = {"frontier", "smart", "balanced", "fast", "local"}
-VALID_MODALITIES = {"text", "image", "audio"}
+VALID_MODALITIES = {"text", "image", "audio", "video", "music"}
 VALID_HAND_CATEGORIES = {
     "communication", "content", "data", "development",
     "devops", "finance", "productivity", "research", "social",