feat: complete release readiness work

- consolidate API ownership and remove dead vendor code

- add Stripe billing, learning paths, and public CV hardening

- add migration, recovery, security, audit, and browser gates

JobTrackerApi/Models/CoverLetterVersion.cs

@@ -0,0 +1,41 @@
+namespace JobTrackerApi.Models;
+
+// Phase 5.4 — Application Assets.
+//
+// Append-only history for a job application's cover letter. JobApplication.CoverLetterText stays the
+// CURRENT text and the existing API contract around it is unchanged; this only records what it used
+// to be, so an AI rewrite or a bad edit is never destructive. Mirrors CvVariantVersion: restore
+// re-saves an old snapshot as a new version rather than rewinding.
+//
+// Reconciler-owned (docs/infrastructure/database-ownership.md) — its migration is a no-op.
+public sealed class CoverLetterVersion
+{
+    public int Id { get; set; }
+    public string OwnerUserId { get; set; } = string.Empty;
+    public int JobApplicationId { get; set; }
+    public JobApplication? JobApplication { get; set; }
+
+    public int Version { get; set; }
+    public string Text { get; set; } = string.Empty;
+
+    // manual | ai | template | restore — how this text came to exist, so the history list can show
+    // whether the user wrote it or approved it from a suggestion.
+    public string Source { get; set; } = "manual";
+
+    // For an AI-sourced version: which action produced it (generate | improve | shorten | expand |
+    // tone | tailor). Null for anything the user typed.
+    public string? AiAction { get; set; }
+
+    public DateTimeOffset CreatedAtUtc { get; set; } = DateTimeOffset.UtcNow;
+}
+
+public static class CoverLetterSources
+{
+    public const string Manual = "manual";
+    public const string Ai = "ai";
+    public const string Template = "template";
+    public const string Restore = "restore";
+
+    public static bool IsValid(string? value) =>
+        value is Manual or Ai or Template or Restore;
+}