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>

Apis/page-fetcher-api/Services/PageFetcherService.cs

@@ -101,6 +101,7 @@ public sealed class PageFetcherService
             Id = Guid.NewGuid().ToString("N"),
             Url = request.Url,
             CallerService = request.CallerService ?? string.Empty,
+            JobSearchSessionId = request.JobSearchSessionId,
             HttpStatusCode = statusCode,
             Html = html,
             Text = text,