Close billing and webhook gaps from the security audit

[skyfallwastaken]

Fixes from a security audit of the gateway. Each fix has a regression test.

Billing

• Fallback models priced. The proxy reserved only for body.model, so a :free model with paid models fallbacks ran on a $0 hold. Every model in models must now be listed, and the hold covers the priciest. Unpriced extras (:online, plugins, web_search_options, server-side tools) add the unknown-model hold.
• Right output cap. /responses now reads only max_output_tokens. Chat Completions takes the larger of max_tokens / max_completion_tokens, and 0 means no cap. Previously max_tokens: 1 could price a request that generated up to the model maximum.
• Abandoned requests are billed. Billed upstream calls no longer take the client's abort signal, because aborting before the response lost the generation id and released the hold unbilled.
  • OpenRouter: the proxy cancels the response itself when the client has gone, so OpenRouter stops the generation and reconciliation bills the partial tokens.
  • OCR, Exa and Jev finish and are charged.
• Long streams keep their hold. A response still being read at half its reservation's lifetime moves to pending_reconciliation, so the expiry sweeper can't release a hold the request still owes.
• Exa / Mistral OCR / TypeSafe. A 2xx without a readable cost now charges the hold (usage_source = fallback). These providers have no reconciliation lookup, so it used to be released uncharged.

Other

• GitHub secret scanning. Signatures are verified as DER with node:crypto. WebCrypto's ECDSA verify rejected every real alert, so leaked keys were never revoked. The test now checks GitHub's published sample.
• Replicate uploads. Uploads still in flight count toward the daily quota, so parallel uploads can't exceed it.
• Framing. frame-ancestors 'none' on pages, so /keys can't be framed.

bun run typecheck and bun test (327 pass) are clean.

🤖 Generated with Claude Code

[skyfallwastaken]

@greptileapps review

[skyfallwastaken]

@greptile review