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/API/Filters/FluentValidationFilter.cs

@@ -0,0 +1,69 @@
+using FluentValidation;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace Extrudex.API.Filters;
+
+/// <summary>
+/// Action filter that automatically validates request DTOs using FluentValidation
+/// validators registered in DI. Runs before the controller action executes.
+/// Returns 400 Bad Request with validation errors if validation fails.
+/// </summary>
+public class FluentValidationFilter : IAsyncActionFilter
+{
+    private readonly IServiceProvider _serviceProvider;
+    private readonly ILogger<FluentValidationFilter> _logger;
+
+    public FluentValidationFilter(IServiceProvider serviceProvider, ILogger<FluentValidationFilter> logger)
+    {
+        _serviceProvider = serviceProvider;
+        _logger = logger;
+    }
+
+    public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
+    {
+        foreach (var argument in context.ActionArguments.Values)
+        {
+            if (argument is null) continue;
+
+            var argumentType = argument.GetType();
+            var validatorType = typeof(IValidator<>).MakeGenericType(argumentType);
+
+            // Try to resolve a validator for this argument type
+            var validator = _serviceProvider.GetService(validatorType) as IValidator;
+            if (validator is null) continue;
+
+            _logger.LogDebug("Validating {Type} with {Validator}", argumentType.Name, validator.GetType().Name);
+
+            var validationResult = await validator.ValidateAsync(
+                new ValidationContext<object>(argument), context.HttpContext.RequestAborted);
+
+            if (!validationResult.IsValid)
+            {
+                foreach (var error in validationResult.Errors)
+                {
+                    context.ModelState.AddModelError(error.PropertyName, error.ErrorMessage);
+                }
+            }
+        }
+
+        if (!context.ModelState.IsValid)
+        {
+            var errors = context.ModelState
+                .Where(kvp => kvp.Value?.Errors.Count > 0)
+                .ToDictionary(
+                    kvp => kvp.Key,
+                    kvp => kvp.Value!.Errors.Select(e => e.ErrorMessage).ToArray());
+
+            context.Result = new BadRequestObjectResult(new
+            {
+                title = "Validation failed",
+                status = 400,
+                errors
+            });
+            return;
+        }
+
+        await next();
+    }
+}