Fix JSON enum serialization and audit log jsonb format

Add JsonStringEnumConverter so string enum values (ServiceCategory,
Priority, PricingMode, LineItemSource) deserialize correctly from
frontend requests. Wrap AuditService detail strings in a JSON object
to satisfy the Postgres jsonb column type. Relax global.json SDK
version to match installed 8.0.1xx feature band.

api/global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "8.0.400",
+    "version": "8.0.100",
     "rollForward": "latestFeature"
   }
 }

api/src/ProposalSystem.Api/Program.cs

@@ -137,6 +137,9 @@ builder.Services.AddValidatorsFromAssemblyContaining<CreateProposalValidator>();
 builder.Services.AddControllers(options =>
 {
     options.Filters.Add<ValidationFilter>();
+}).AddJsonOptions(options =>
+{
+    options.JsonSerializerOptions.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());
 });
 
 // Middleware

api/src/ProposalSystem.Infrastructure/Services/AuditService.cs

@@ -1,3 +1,4 @@
+using System.Text.Json;
 using ProposalSystem.Application.Interfaces;
 using ProposalSystem.Domain.Entities;
 using ProposalSystem.Infrastructure.Data;
@@ -17,13 +18,17 @@ public class AuditService : IAuditService
 
     public async Task LogAsync(AuditAction action, Guid? proposalId, string? details = null, CancellationToken ct = default)
     {
+        var jsonDetails = details != null
+            ? JsonSerializer.Serialize(new { message = details })
+            : null;
+
         var entry = new AuditLog
         {
             Id = Guid.NewGuid(),
             ProposalId = proposalId,
             UserId = _currentUser.UserId,
             Action = action,
-            Details = details,
+            Details = jsonDetails,
             Timestamp = DateTime.UtcNow,
             IpAddress = _currentUser.IpAddress,
         };