codeSplitters limits: rules (and queueing) for preview deploys and dependency-doctor runs #279

Follow-up from #268, which adds the queue on-fail action and allows it only on digs.

#268 also named preview deploys (`POST /api/repos/:o/:r/preview`) and dependency-doctor runs (`POST /api/repos/:o/:r/deps/run`), but neither has a rate-limit rule yet, so there's nothing to queue. Adding them is:

• a rule each in `RULES` (`src/limits.ts`) with `queueable: true`, and a `ruleFor` case;
• choosing default caps;
• checking that their answers survive a replay: the doctor's run can answer 202 `{running: true}` and the page polls the doctor itself, and a preview run streams its stages.

Cloudflare Queues stays an option if RustyBuns gets a `queue` binding (ROADMAP: 🔲 on every target).

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