discovery: add a SmartRecruiters public board source

[LXBStudioLLC]

Envoy's job search reads public ATS board APIs (Greenhouse, Lever, Ashby, Workable, Recruitee). SmartRecruiters is another common ATS with a public postings API, and the JobSource.SmartRecruiters enum value already exists, so this is a well-scoped "follow the existing pattern" task.

What to do

1. Add src/Envoy.Discovery/Sources/SmartRecruitersSource.cs implementing IAtsBoardSource, following the shape of GreenhouseSource and LeverSource.
   • Public endpoint: https://api.smartrecruiters.com/v1/companies/{companyIdentifier}/postings (public postings, no auth). Paginated with limit and offset.
   • Map each posting to a JobPosting (title, location, company, url, posted date, RawSourceId). Use the shared Envoy.Discovery.Internal helpers (Json, HtmlText, DateParsing, Naming) like the other sources do.
   • Plain GET, short timeout, no scraping, no auth. Stay within the read-public-data-only rule.
2. Register it in ServiceRegistration (AddEnvoyDiscovery) so JobDiscoveryService picks it up.
3. Add a SmartRecruiters entry (a real public company token) to the seed boards (SeedBoards.cs, seed-boards.json) so it works out of the box.
4. Add tests in Envoy.Discovery.Tests with a stubbed HttpClient (see the existing AtsSourceTests and StubHttpMessageHandler). No real network calls.

Acceptance criteria

• SmartRecruitersSource : IAtsBoardSource with Ats => JobSource.SmartRecruiters
• Hits only the public SmartRecruiters postings API. No auth, no scraping
• Registered in AddEnvoyDiscovery. A seed board added
• Handles empty or error responses without throwing the whole scan (return an empty list)
• Tests with a stubbed HttpClient. No real network calls
• dotnet test passes, including DiscoveryContainerResolutionTests
• PR targets main

A bit more involved than the signal issues, but a clean copy-the-pattern task.