feat(career): CV builder backend — data-driven theme engine + variant model

Phase 4 foundation. A CvVariant is a lens over the master CareerProfile
(section order/visibility, per-item overrides keyed by ItemKey, theme +
builder settings) — it references career data, never duplicates it. One
renderer (ThemedCvRenderer) draws every theme; a theme is pure data
(CvThemeCatalog, 8 professional themes), so adding a theme needs no renderer
change. Autosave version history + non-destructive restore, public CV via
/api/public-cv/{slug} (anonymous, noindex, filter-bypassing owner load), and
an AI-assist endpoint reusing the existing provider abstraction (suggestions
only, never auto-applied).

- Models: CvVariant/CvVariantVersion, CvVariantSettings, CvTheme + catalog
- Services: CvVariantResolver (profile+lens -> render model), ThemedCvRenderer,
  CvVariantService, CareerProfileService.LoadStructuredForOwnerAsync (public)
- API: CvVariantController (/api/cv), PublicCvController (/api/public-cv)
- Migration AddCvVariants (2 self-contained tables; verified applied on the
  running container), 16 tests (resolver/renderer/service), 296 backend green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

JobTrackerApi/Migrations/JobTrackerContextModelSnapshot.cs

@@ -795,6 +795,89 @@ namespace JobTrackerApi.Migrations
                     b.ToTable("CvUploadArtifacts");
                 });
 
+            modelBuilder.Entity("JobTrackerApi.Models.CvVariant", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER");
+
+                    b.Property<DateTimeOffset>("CreatedAtUtc")
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("IsPublic")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<int?>("JobApplicationId")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("OwnerUserId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PublicSlug")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("SettingsJson")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<DateTimeOffset>("UpdatedAtUtc")
+                        .HasColumnType("TEXT");
+
+                    b.Property<int>("Version")
+                        .HasColumnType("INTEGER");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("JobApplicationId");
+
+                    b.HasIndex("PublicSlug")
+                        .IsUnique();
+
+                    b.HasIndex("OwnerUserId", "UpdatedAtUtc");
+
+                    b.ToTable("CvVariants");
+                });
+
+            modelBuilder.Entity("JobTrackerApi.Models.CvVariantVersion", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER");
+
+                    b.Property<DateTimeOffset>("CreatedAtUtc")
+                        .HasColumnType("TEXT");
+
+                    b.Property<int>("CvVariantId")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("OwnerUserId")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("SettingsJson")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Source")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<int>("Version")
+                        .HasColumnType("INTEGER");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("CvVariantId", "Version");
+
+                    b.ToTable("CvVariantVersions");
+                });
+
             modelBuilder.Entity("JobTrackerApi.Models.GmailConnection", b =>
                 {
                     b.Property<int>("Id")
@@ -1803,6 +1886,27 @@ namespace JobTrackerApi.Migrations
                     b.Navigation("Artifact");
                 });
 
+            modelBuilder.Entity("JobTrackerApi.Models.CvVariant", b =>
+                {
+                    b.HasOne("JobTrackerApi.Models.JobApplication", "JobApplication")
+                        .WithMany()
+                        .HasForeignKey("JobApplicationId")
+                        .OnDelete(DeleteBehavior.SetNull);
+
+                    b.Navigation("JobApplication");
+                });
+
+            modelBuilder.Entity("JobTrackerApi.Models.CvVariantVersion", b =>
+                {
+                    b.HasOne("JobTrackerApi.Models.CvVariant", "CvVariant")
+                        .WithMany()
+                        .HasForeignKey("CvVariantId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("CvVariant");
+                });
+
             modelBuilder.Entity("JobTrackerApi.Models.InterviewPrepNote", b =>
                 {
                     b.HasOne("JobTrackerApi.Models.JobApplication", "JobApplication")