fix(rd21): guard career and job saves with durable revisions

JobTrackerApi/Services/RevisionConflictExceptionHandler.cs

@@ -0,0 +1,22 @@
+using Microsoft.AspNetCore.Diagnostics;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+
+namespace JobTrackerApi.Services;
+
+public sealed class RevisionConflictExceptionHandler : IExceptionHandler
+{
+    public async ValueTask<bool> TryHandleAsync(HttpContext context, Exception exception, CancellationToken cancellationToken)
+    {
+        if (exception is not DbUpdateConcurrencyException) return false;
+        context.Response.StatusCode = StatusCodes.Status409Conflict;
+        await context.Response.WriteAsJsonAsync(new ProblemDetails
+        {
+            Status = StatusCodes.Status409Conflict,
+            Title = "The record changed",
+            Detail = "Your draft was not saved. Reload and review the current record before saving again.",
+            Extensions = { ["code"] = "revision_conflict" },
+        }, cancellationToken);
+        return true;
+    }
+}