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/Validators/FilamentValidators.cs

@@ -0,0 +1,108 @@
+using Extrudex.API.DTOs.Filaments;
+using FluentValidation;
+
+namespace Extrudex.API.Validators;
+
+/// <summary>
+/// Validation rules for creating a Filament (Spool) via the /filaments route.
+/// Mirrors the domain rules enforced in the controller and ensures consistent
+/// validation regardless of the request pipeline entry point.
+/// </summary>
+public class CreateFilamentRequestValidator : AbstractValidator<CreateFilamentRequest>
+{
+    public CreateFilamentRequestValidator()
+    {
+        RuleFor(x => x.MaterialBaseId)
+            .NotEmpty().WithMessage("MaterialBaseId is required.");
+
+        RuleFor(x => x.MaterialFinishId)
+            .NotEmpty().WithMessage("MaterialFinishId is required.");
+
+        RuleFor(x => x.Brand)
+            .NotEmpty().WithMessage("Brand is required.")
+            .MaximumLength(200).WithMessage("Brand must not exceed 200 characters.");
+
+        RuleFor(x => x.ColorName)
+            .NotEmpty().WithMessage("ColorName is required.")
+            .MaximumLength(200).WithMessage("ColorName must not exceed 200 characters.");
+
+        RuleFor(x => x.ColorHex)
+            .NotEmpty().WithMessage("ColorHex is required.")
+            .Matches(@"^#[0-9A-Fa-f]{6}$").WithMessage("ColorHex must be a valid hex color code (e.g., #FF0000).");
+
+        RuleFor(x => x.WeightTotalGrams)
+            .GreaterThan(0).WithMessage("Total weight must be greater than zero.");
+
+        RuleFor(x => x.WeightRemainingGrams)
+            .GreaterThanOrEqualTo(0).WithMessage("Remaining weight must be non-negative.");
+
+        RuleFor(x => x.WeightRemainingGrams)
+            .LessThanOrEqualTo(x => x.WeightTotalGrams)
+            .WithMessage("WeightRemainingGrams cannot exceed WeightTotalGrams.");
+
+        RuleFor(x => x.FilamentDiameterMm)
+            .GreaterThan(0).WithMessage("Filament diameter must be greater than zero.");
+
+        RuleFor(x => x.SpoolSerial)
+            .NotEmpty().WithMessage("SpoolSerial is required.")
+            .MaximumLength(200).WithMessage("SpoolSerial must not exceed 200 characters.");
+
+        When(x => x.PurchasePrice.HasValue, () =>
+        {
+            RuleFor(x => x.PurchasePrice!.Value)
+                .GreaterThanOrEqualTo(0).WithMessage("Purchase price must be non-negative.");
+        });
+    }
+}
+
+/// <summary>
+/// Validation rules for updating a Filament (Spool) via the /filaments route.
+/// Enforces the same domain rules as creation, plus ensures the updated
+/// WeightRemainingGrams does not exceed the updated WeightTotalGrams.
+/// </summary>
+public class UpdateFilamentRequestValidator : AbstractValidator<UpdateFilamentRequest>
+{
+    public UpdateFilamentRequestValidator()
+    {
+        RuleFor(x => x.MaterialBaseId)
+            .NotEmpty().WithMessage("MaterialBaseId is required.");
+
+        RuleFor(x => x.MaterialFinishId)
+            .NotEmpty().WithMessage("MaterialFinishId is required.");
+
+        RuleFor(x => x.Brand)
+            .NotEmpty().WithMessage("Brand is required.")
+            .MaximumLength(200).WithMessage("Brand must not exceed 200 characters.");
+
+        RuleFor(x => x.ColorName)
+            .NotEmpty().WithMessage("ColorName is required.")
+            .MaximumLength(200).WithMessage("ColorName must not exceed 200 characters.");
+
+        RuleFor(x => x.ColorHex)
+            .NotEmpty().WithMessage("ColorHex is required.")
+            .Matches(@"^#[0-9A-Fa-f]{6}$").WithMessage("ColorHex must be a valid hex color code (e.g., #FF0000).");
+
+        RuleFor(x => x.WeightTotalGrams)
+            .GreaterThan(0).WithMessage("Total weight must be greater than zero.");
+
+        RuleFor(x => x.WeightRemainingGrams)
+            .GreaterThanOrEqualTo(0).WithMessage("Remaining weight must be non-negative.");
+
+        RuleFor(x => x.WeightRemainingGrams)
+            .LessThanOrEqualTo(x => x.WeightTotalGrams)
+            .WithMessage("WeightRemainingGrams cannot exceed WeightTotalGrams.");
+
+        RuleFor(x => x.FilamentDiameterMm)
+            .GreaterThan(0).WithMessage("Filament diameter must be greater than zero.");
+
+        RuleFor(x => x.SpoolSerial)
+            .NotEmpty().WithMessage("SpoolSerial is required.")
+            .MaximumLength(200).WithMessage("SpoolSerial must not exceed 200 characters.");
+
+        When(x => x.PurchasePrice.HasValue, () =>
+        {
+            RuleFor(x => x.PurchasePrice!.Value)
+                .GreaterThanOrEqualTo(0).WithMessage("Purchase price must be non-negative.");
+        });
+    }
+}