UsageEvent emission missing on /v1/images/* (#226 follow-up)

[moonming]

Follow-up from #226 MVP (PR #402 lands /v1/embeddings).

Problem

`/v1/images/generations`, `/v1/images/edits`, `/v1/images/variations` (OpenAI DALL-E shape image endpoints) do not emit `UsageEvent` to `ProxyState.usage_sink` on success. Same budget-ledger / /logs gap as embeddings.

Image endpoints are billed per-image (not per-token), and the response shape does not include a usage block. The `UsageEvent` shape will need adapting:

• `prompt_tokens` doesn't map. The natural counter is `n` (number of images requested) and the resolution / quality tier.
• Cost calculation cp-api-side relies on pricing catalog keyed on (model, resolution, quality) — DP only needs to surface enough metadata for that lookup.

Acceptance criteria

• On a successful 200 from /v1/images/*, exactly one `UsageEvent` arrives on the configured sink
• Image-count / resolution mapping documented in the PR (likely needs cp-api coordination)
• `model_id`, `api_key_id`, `inbound_protocol = "openai"` per chat.rs convention

References

• Parent issue: Non-chat endpoints don't emit UsageEvents — cp-api's spend ledger misses spend from /v1/responses, /v1/embeddings, /v1/audio*, /v1/images*, /v1/rerank #226
• MVP PR: feat(embeddings): emit UsageEvent on /v1/embeddings 200 (#226 MVP) #402 (embeddings)
• OpenAI images API spec: https://platform.openai.com/docs/api-reference/images
• Existing pattern: `crates/aisix-proxy/src/chat.rs::emit_usage_event`

[coderabbitai]

🔗 Related PRs

#355 - fix(proxy): use UUID request IDs across handlers [merged]
#363 - fix(proxy): record messages stream telemetry [merged]
#378 - feat(proxy): log routing target attempts [merged]

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