Bulk create check rules

Atomically create up to 1000 Prometheus alert rules in a single request. Create-only — every item produces a brand-new check rule. The public schema uses id as the user-defined origin; per-item resolution priority mirrors the single-rule POST handler (id → server-generated UUID with "api-" prefix). Any item whose resolved origin or id collides with an existing rule (including soft-deleted rules whose origin is still reserved by the unique constraint) is rejected with a per-item diagnostic.

If any item fails validation (PromQL, intervals, summary length, template functions, origin/id collision, etc.) the whole batch is rejected with a 400 that carries per-item diagnostics in the message; nothing is written. On any DB failure the entire batch is rolled back and 500 is returned. The endpoint is not idempotent — a successful batch retried verbatim will 400 on every item the second time.

This endpoint accepts request bodies up to 2 MB (vs. the 256 KB default for other endpoints) to accommodate batched payloads of up to 1000 rules per request.