Improved source filter with multi-source matching

The problem: When filtering jobs by source domain, only the direct URL was checked. If a job was posted on a company career page but discovered through a job board like LinkedIn or Indeed, filtering by that job board wouldn't return it. You also had no visibility into how many jobs each source had.

The solution: The source filter now matches against both the job URL and the original source URL. When typing a domain, you get autocomplete suggestions showing the number of jobs per source—so you can quickly find and pick the right one.

• Multi-source matching — Filtering by "linkedin.com" now returns jobs originally discovered on LinkedIn, even if the job URL points to the company's career page.
• Job count suggestions — Domain suggestions show how many jobs are associated with each source, helping you pick the most relevant one.
• source_url in API responses — The job API now returns a source_url field alongside url, so you can see both where the job lives and where it was found.