fix(lever): paginate public postings API

[pashawkola33]

Summary

• fetch Lever postings in bounded skip/limit pages instead of one unbounded tenant response
• cap pagination at 40 pages (2,000 postings at page size 50)
• preserve provider order while deduplicating repeated posting IDs across page boundaries
• discard partial tenant results if a later page fails, preserving tenant isolation and health monitoring semantics
• reject malformed/non-array pages and pathological boards that ignore skip

Why

Large Lever boards can exceed the shared HTTP response-size bound when fetched as a single array. Pagination fixes that at the source without increasing the global transport limit.

Verification

• focused LeverJobSourceTest: 12 tests, 0 failures/errors
• targeted regression suite: 88 tests, 0 failures/errors/skips
• git diff --check clean

Full verification is delegated to GitHub Actions.