feat: introduce CV variant schema, dual-written from tailored CV saves

Phase F2 of the Career Workspace roadmap: CvVariant, CvVersion, and
TailoredApplication -- the reference seam. Per the product boundary,
a job application REFERENCES a tailored output; it does not own it.
CvVariant is not job-owned: it survives job deletion (SetNull on its
optional CareerProfile link, not cascaded), can be reused across
applications, and carries its own append-only CvVersion history.
TailoredApplication is the join that links a variant to a job
(cascades with either side, since the link is meaningless without
both).

Rather than shipping empty tables with no consumer, this dual-writes
from both existing TailoredCvDraft save paths (SaveTailoredCvDraft,
UpsertGeneratedTailoredCvDraftAsync via GenerateTailoredCvDraft) --
same pattern as CareerProfile in Phase F1. TailoredCvDraft remains
authoritative for every existing read path; the sync is additive and
never blocks or fails a draft save.

2 new tests: variant/version/link created on first save, same variant
reused (not duplicated) with version incrementing on subsequent
saves. Verified against the real dev DB -- FK dependency ordering
(CareerProfiles -> CvVariants -> CvVersions/TailoredApplications)
holds in both SQLite and MySQL reconciler dialects.

JobTrackerApi.Tests/CvVariantSyncTests.cs

@@ -0,0 +1,105 @@
+using System.Security.Claims;
+using JobTrackerApi.Controllers;
+using JobTrackerApi.Data;
+using JobTrackerApi.Models;
+using JobTrackerApi.Services;
+using JobTrackerApi.Tests.TestSupport;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging.Abstractions;
+using Moq;
+using Xunit;
+
+namespace JobTrackerApi.Tests;
+
+// Career Workspace foundation, Phase F2 (career-workspace-implementation-roadmap.md). Every
+// TailoredCvDraft save should dual-write a CvVariant + CvVersion + TailoredApplication without
+// changing TailoredCvDraft's own behavior -- these tests lock in that seam.
+public sealed class CvVariantSyncTests
+{
+    [Fact]
+    public async Task SaveTailoredCvDraft_creates_a_variant_version_and_application_link()
+    {
+        await using var db = TestHostFactory.CreateInMemoryDb();
+        var job = await SeedJobAsync(db);
+        var controller = CreateController(db, "user-1");
+
+        var result = await controller.SaveTailoredCvDraft(job.Id, new JobApplicationsController.SaveTailoredCvDraftRequest(
+            "ats-minimal", "Backend Engineer", new List<string> { "Built things." }, new List<string> { "C#" },
+            new List<TailoredCvExperienceItem>(), new List<TailoredCvEducationItem>(), new List<TailoredCvCustomSection>(), null, "edited"), CancellationToken.None);
+
+        Assert.IsType<NoContentResult>(result);
+
+        var variant = Assert.Single(db.CvVariants.IgnoreQueryFilters().Where(x => x.OwnerUserId == "user-1"));
+        Assert.Equal(1, variant.Version);
+        Assert.Contains("Backend Developer", variant.Name);
+
+        var link = Assert.Single(db.TailoredApplications.IgnoreQueryFilters().Where(x => x.JobApplicationId == job.Id));
+        Assert.Equal(variant.Id, link.CvVariantId);
+
+        var version = Assert.Single(db.CvVersions.IgnoreQueryFilters().Where(x => x.CvVariantId == variant.Id));
+        Assert.Equal(1, version.Version);
+    }
+
+    [Fact]
+    public async Task SaveTailoredCvDraft_reuses_the_same_variant_on_subsequent_saves()
+    {
+        await using var db = TestHostFactory.CreateInMemoryDb();
+        var job = await SeedJobAsync(db);
+        var controller = CreateController(db, "user-1");
+
+        var request = new JobApplicationsController.SaveTailoredCvDraftRequest(
+            "ats-minimal", "Backend Engineer", new List<string> { "Built things." }, new List<string> { "C#" },
+            new List<TailoredCvExperienceItem>(), new List<TailoredCvEducationItem>(), new List<TailoredCvCustomSection>(), null, "edited");
+
+        await controller.SaveTailoredCvDraft(job.Id, request, CancellationToken.None);
+        await controller.SaveTailoredCvDraft(job.Id, request with { Headline = "Updated headline" }, CancellationToken.None);
+
+        var variant = Assert.Single(db.CvVariants.IgnoreQueryFilters().Where(x => x.OwnerUserId == "user-1"));
+        Assert.Equal(2, variant.Version);
+
+        var links = db.TailoredApplications.IgnoreQueryFilters().Where(x => x.JobApplicationId == job.Id).ToList();
+        Assert.Single(links);
+
+        var versions = db.CvVersions.IgnoreQueryFilters().Where(x => x.CvVariantId == variant.Id).ToList();
+        Assert.Equal(2, versions.Count);
+    }
+
+    private static async Task<JobApplication> SeedJobAsync(JobTrackerContext db)
+    {
+        var company = new Company { Name = "Acme", OwnerUserId = "user-1" };
+        db.Companies.Add(company);
+        db.Users.Add(new ApplicationUser { Id = "user-1", UserName = "user@example.test", Email = "user@example.test" });
+        await db.SaveChangesAsync();
+
+        var job = new JobApplication { JobTitle = "Backend Developer", CompanyId = company.Id, OwnerUserId = "user-1" };
+        db.JobApplications.Add(job);
+        await db.SaveChangesAsync();
+        return job;
+    }
+
+    private static JobApplicationsController CreateController(JobTrackerContext db, string userId)
+    {
+        var user = db.Users.AsNoTracking().FirstOrDefault(x => x.Id == userId);
+        var controller = new JobApplicationsController(
+            db,
+            Mock.Of<ISummarizerService>(),
+            Mock.Of<IAppEmailSender>(),
+            TestHostFactory.CreateUserManager(user).Object,
+            NullLogger<JobApplicationsController>.Instance,
+            Mock.Of<ICvTemplateRenderer>(),
+            Mock.Of<ICvPdfExporter>());
+        controller.ControllerContext = new ControllerContext
+        {
+            HttpContext = new DefaultHttpContext
+            {
+                User = new ClaimsPrincipal(new ClaimsIdentity(new[]
+                {
+                    new Claim(ClaimTypes.NameIdentifier, userId)
+                }, "test"))
+            }
+        };
+        return controller;
+    }
+}