fix: close release preflight gaps

Route public health checks to the API, backfill and synchronize job opportunities, stabilize SPA smoke tests, and document operator-only production steps.

JobTrackerApi.Tests/GmailControllerTests.cs

@@ -816,6 +816,10 @@ public sealed class GmailControllerTests
         Assert.Equal(1, created.Imported);
         Assert.Equal("thread-suggested", created.ThreadId);
         Assert.Equal(1, await db.JobApplications.CountAsync());
+        var linkedOpportunity = await db.JobApplications.Include(application => application.Job).SingleAsync();
+        Assert.NotNull(linkedOpportunity.JobId);
+        Assert.Equal("Platform Engineer", linkedOpportunity.Job!.JobTitle);
+        Assert.Equal("gmail", linkedOpportunity.Job.Source);
         Assert.Equal(1, await db.Correspondences.CountAsync());
     }
 

JobTrackerApi.Tests/JobOpportunitySyncTests.cs

@@ -0,0 +1,38 @@
+using JobTrackerApi.Models;
+using JobTrackerApi.Services;
+using JobTrackerApi.Tests.TestSupport;
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+
+namespace JobTrackerApi.Tests;
+
+public sealed class JobOpportunitySyncTests
+{
+    [Fact]
+    public async Task Backfill_links_each_legacy_application_once()
+    {
+        await using var db = TestHostFactory.CreateInMemoryDb();
+        var company = new Company { OwnerUserId = "user-1", Name = "Acme" };
+        db.Companies.Add(company);
+        await db.SaveChangesAsync();
+        db.JobApplications.Add(new JobApplication
+        {
+            OwnerUserId = "user-1",
+            CompanyId = company.Id,
+            JobTitle = "Platform Engineer",
+            Location = "Oslo",
+            SavedAt = new DateTime(2026, 7, 1, 10, 0, 0, DateTimeKind.Utc),
+        });
+        await db.SaveChangesAsync();
+
+        Assert.Equal(1, await JobOpportunitySync.BackfillLegacyAsync(db));
+        Assert.Equal(0, await JobOpportunitySync.BackfillLegacyAsync(db));
+
+        var application = await db.JobApplications.IgnoreQueryFilters().Include(item => item.Job).SingleAsync();
+        Assert.NotNull(application.JobId);
+        Assert.Equal("Platform Engineer", application.Job!.JobTitle);
+        Assert.Equal("Oslo", application.Job.Location);
+        Assert.Equal("legacy", application.Job.Source);
+        Assert.Equal(1, await db.Jobs.IgnoreQueryFilters().CountAsync());
+    }
+}