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-data/PageFetchDbContext.cs

@@ -36,8 +36,11 @@ public sealed class PageFetchDbContext : DbContext
             entity.Property(x => x.Html).IsRequired();
             entity.Property(x => x.Text).IsRequired();
             entity.Property(x => x.ErrorMessage).HasMaxLength(2000);
+            entity.Property(x => x.JobSearchSessionId).HasMaxLength(64);
             entity.Property(x => x.CreatedAt).HasDefaultValueSql("SYSUTCDATETIME()");
 
+            entity.HasIndex(x => x.JobSearchSessionId);
+
             entity.HasIndex(x => x.Url);
             entity.HasIndex(x => x.CreatedAt);
         });