Link pageFetcher.PageFetches to cvSearch.JobSearchSessions

Adds nullable JobSearchSessionId to PageFetchEntity and FetchPageRequest.
cv-search-job passes session.Id on every fetch so all Playwright page
loads for a job search session can be traced back to their session.
Includes index on JobSearchSessionId for efficient lookup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jobs/cv-search-job/Tasks/CvSearchJobTask.cs

@@ -169,7 +169,8 @@ public sealed class CvSearchJobTask : IJobTask
                 {
                     Url = url,
                     WaitFor = "domcontentloaded",
-                    CallerService = "cv-search-job"
+                    CallerService = "cv-search-job",
+                    JobSearchSessionId = session.Id
                 }, ct);
 
                 if (!fetchResponse.Success || string.IsNullOrWhiteSpace(fetchResponse.Text))