feat(minimax): add per_call_cost for video and music models (#79)

Video and music modality models are billed per generation rather than
per token. Without this field the librefang runtime metering layer
records every call as $0 and emits a warning. Sourced from
https://platform.minimax.io/docs/guides/pricing-paygo.

- Hailuo 2.3 Fast:  $0.33 per call (1080P/6s upper bound)
- Hailuo 2.3:       $0.56 per call (1080P/6s upper bound)
- Hailuo 02:        $0.56 per call (1080P/6s upper bound)
- Music 2.6:        $0.15 per up-to-5-minute track
- Lyrics gen:       $0.01 per song

Also adds per_call_cost to schema.toml so the field is documented
alongside the other cost fields.

schema.toml

@@ -116,6 +116,12 @@ required = false
 description = "USD per million image output tokens (for image-generation models)"
 example = 30.0
 
+[provider.sections.models.fields.per_call_cost]
+type = "number"
+required = false
+description = "Flat USD cost per generation, for models billed per call rather than per token. Strongly recommended for modality='video' and modality='music' (which have no per-token pricing); may also be used for image/audio models that quote a flat per-image or per-request rate. When absent for a per-call modality, the runtime metering layer records the call as $0 and logs a warning."
+example = 0.33
+
 [provider.sections.models.fields.last_verified]
 type = "string"
 required = false