UsageEvent: carry non-token cost basis for audio (duration) and images (n×size×quality)

Context

#226 (non-chat UsageEvent emission) is now complete — all 6 endpoints emit (embeddings/responses/completions/rerank #402/#426/#425/#428, audio/images #454). But the audio/images MVP (#406/#407) only carries token counts. The token shape covers the modern models (gpt-image-1, gpt-4o-transcribe), which return a usage token block. The legacy models do not, and are not billed by tokens at all:

• audio (whisper-1, and gpt-4o-transcribe under some response_formats) → billed by duration-seconds (OpenAI returns usage:{type:"duration",seconds:N}).
• images (dall-e-2/3) → billed by (model, size, quality) × n — no token block at all.

Today those requests emit a zero-token UsageEvent: visible + attributed in the ledger (no longer invisible — that gap is closed by #454), but with no cost basis, so cp-api cannot price them.

Gap

The UsageEvent wire struct (aisix-obs) has no field to carry duration-seconds or image (count/size/quality). cp-api's pricing therefore can't compute spend for legacy audio/image traffic even though the event now arrives.

Proposed work (cross-repo)

1. ai-gateway — extend UsageEvent with optional cost-basis fields, e.g. audio_duration_seconds: Option<f64> and image_count/image_size/image_quality: Option<...> (all #[serde(skip_serializing_if)] so existing token-only events are unchanged on the wire). Populate them in audio.rs / images.rs from the response usage.duration and the request n/size/quality.
2. cp-api (AISIX-Cloud) — add pricing rules keyed on duration-seconds (audio) and (model,size,quality) tuples (images), and apply them when the new fields are present.

Cross-checked against comparable open-source multi-provider gateways: audio is priced cost_per_second × duration; images via a (quality,size,n) cost calculator — confirming duration + (n,size,quality) are the right cost dimensions.

Acceptance

• UsageEvent carries duration (audio) + n/size/quality (images), back-compatible on the wire
• DP populates them for legacy whisper-1 / dall-e-* traffic
• cp-api prices duration-based audio and per-image image spend
• e2e asserts a dall-e-3 / whisper-1 request produces a priced (non-zero spend) ledger entry

Refs: #226 (parent, now complete), #454 (origin — the token-MVP that deferred this)

Start with the UsageEvent wire struct and trace the audio.rs and images.rs paths that emit events. Then inspect the cp-api pricing rules and the end-to-end ledger coverage for legacy whisper-1 and dall-e-* requests. Done means duration and image dimensions remain wire-compatible, are populated, and produce non-zero priced ledger entries.