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>

Models/CvVariantSettings.cs

@@ -0,0 +1,87 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace JobTrackerApi.Models;
+
+// The "lens" a CvVariant stores. Deliberately all-optional + defaulted so an empty settings object
+// renders a sensible default CV straight from the master profile. Overrides are keyed by the master
+// profile's stable ItemKeys so career data is referenced, never copied.
+public sealed class CvVariantSettings
+{
+    public string ThemeId { get; set; } = "modern";
+
+    // Theme overrides (null => use the theme's own value).
+    public string? AccentColor { get; set; }
+    public string? HeadingFont { get; set; }
+    public string? BodyFont { get; set; }
+    public string? Density { get; set; }      // compact | balanced | roomy
+    public string? PageSize { get; set; }     // a4 | letter
+    public string? DateFormat { get; set; }   // display hint for the frontend; renderer uses source strings
+    public string? Language { get; set; }
+    public string? Headline { get; set; }     // override the contact headline for this variant
+
+    public bool ShowPhoto { get; set; }
+    public bool ShowPageNumbers { get; set; }
+    public bool ShowIcons { get; set; } = true;
+
+    // Section order + visibility + optional renamed heading. Empty => default order, all visible.
+    public List<CvSectionSetting> Sections { get; set; } = new();
+
+    // Per-item overrides keyed by ItemKey (job/education/project/certification stable id).
+    public Dictionary<string, CvItemOverride> Overrides { get; set; } = new();
+
+    // Extra sections unique to this variant (not in the master profile).
+    public List<CvCustomSectionSetting> CustomSections { get; set; } = new();
+}
+
+public sealed class CvSectionSetting
+{
+    public string Key { get; set; } = string.Empty; // summary|skills|experience|education|projects|certifications|languages|interests|links|custom:<k>
+    public bool Hidden { get; set; }
+    public string? Title { get; set; } // renamed heading
+}
+
+public sealed class CvItemOverride
+{
+    public bool Hidden { get; set; }
+    public string? Title { get; set; }
+    public string? Subtitle { get; set; }
+    public List<string>? Bullets { get; set; } // null => use master bullets; non-null => replace for this variant only
+}
+
+public sealed class CvCustomSectionSetting
+{
+    public string Key { get; set; } = string.Empty;
+    public string? Title { get; set; }
+    public List<string> Items { get; set; } = new();
+    public bool Hidden { get; set; }
+}
+
+public static class CvVariantSettingsJson
+{
+    private static readonly JsonSerializerOptions Options = new(JsonSerializerDefaults.Web)
+    {
+        PropertyNameCaseInsensitive = true,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+    };
+
+    public static string Serialize(CvVariantSettings? settings) =>
+        JsonSerializer.Serialize(Normalize(settings), Options);
+
+    public static CvVariantSettings Deserialize(string? json)
+    {
+        if (string.IsNullOrWhiteSpace(json)) return new CvVariantSettings();
+        try { return Normalize(JsonSerializer.Deserialize<CvVariantSettings>(json, Options)); }
+        catch { return new CvVariantSettings(); }
+    }
+
+    public static CvVariantSettings Normalize(CvVariantSettings? s)
+    {
+        s ??= new CvVariantSettings();
+        s.ThemeId = string.IsNullOrWhiteSpace(s.ThemeId) ? "modern" : s.ThemeId.Trim().ToLowerInvariant();
+        s.Sections ??= new();
+        s.Overrides ??= new();
+        s.CustomSections ??= new();
+        return s;
+    }
+}