CUB-30: Implement PUT /filaments/{id} update endpoint

- Add FluentValidation validators for CreateFilamentRequest and UpdateFilamentRequest
  with comprehensive validation rules (required fields, string lengths, hex color format,
  weight constraints including WeightRemainingGrams <= WeightTotalGrams, purchase price range)
- Add FluentValidationFilter action filter that auto-runs FluentValidation validators
  for all API controller actions before execution, returning 400 with structured error details
- Register FluentValidationFilter in DI and add it to MVC controller filters in Program.cs
- PUT endpoint was already implemented in FilamentsController with proper validation,
  404 handling, FK existence checks, serial uniqueness check, and weight constraint check
- This change ensures FluentValidation rules are enforced consistently via the pipeline

backend/Program.cs

@@ -1,4 +1,5 @@
 using System.Reflection;
+using Extrudex.API.Filters;
 using Extrudex.API.Hubs;
 using Extrudex.Domain.Interfaces;
 using Extrudex.Infrastructure.Data;
@@ -23,7 +24,10 @@ builder.Services.AddDbContext<ExtrudexDbContext>(options =>
     options.UseNpgsql(connectionString));
 
 // ── API Services ───────────────────────────────────────────
-builder.Services.AddControllers();
+builder.Services.AddControllers(options =>
+{
+    options.Filters.AddService<FluentValidationFilter>();
+});
 builder.Services.AddEndpointsApiExplorer();
 builder.Services.AddSwaggerGen(c =>
 {
@@ -50,6 +54,10 @@ builder.Services.AddSingleton<IQrCodeService, QrCodeService>();
 // Registers all validators from the API assembly into DI.
 builder.Services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());
 
+// Register the FluentValidation action filter so validators run automatically
+// on all API controller actions before the action executes.
+builder.Services.AddScoped<FluentValidationFilter>();
+
 // ── CORS (kiosk + remote browser) ─────────────────────────
 // AllowAnyOrigin disallows credentials by spec; this is fine for
 // REST API calls. SignalR WebSockets negotiate without credentials