feat (optional): Durable Object for exact in-flight concurrency limit

[lex00]

The rate limiter (`src/limit.ts`) has a global per-minute breaker over KV. For exact in-flight concurrency capping (shed when N audits are running concurrently), a Durable Object is the right tool. Defense-in-depth; the per-minute breaker covers the abuse case today.

Acceptance

• A DO caps concurrent/queued audits; excess gets 429.
• Falls back gracefully if the DO is unavailable.