CUB-113: implement core CRUD API endpoints

- Add dtos package with request/response structs
- Add repositories: Material, Filament, Printer, PrintJob, UsageLog
- Add services: FilamentService, PrinterService, PrintJobService
- Add handlers for all 5 resources with consistent error responses
- Wire all endpoints into Chi router under /api
- Validation on POST/PUT filament endpoints
- Filter/pagination support on list endpoints
- Soft-delete for filaments (DELETE /api/filaments/{id})
- go build ./... && go vet ./... → PASS

backend/internal/services/validation.go

@@ -0,0 +1,74 @@
+package services
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/dtos"
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/models"
+)
+
+// colorHexPattern validates hex color strings like #FF0000 or #ff0000.
+var colorHexPattern = regexp.MustCompile(`^#[0-9A-Fa-f]{6}$`)
+
+// validateFilamentSpool performs validation on a FilamentSpool entity.
+// Returns a descriptive error on failure.
+func validateFilamentSpool(s *models.FilamentSpool) error {
+	if s.Name == "" {
+		return errors.New("name is required")
+	}
+	if s.MaterialBaseID <= 0 {
+		return errors.New("material_base_id is required")
+	}
+	if s.MaterialFinishID <= 0 {
+		return errors.New("material_finish_id is required")
+	}
+	if !colorHexPattern.MatchString(s.ColorHex) {
+		return fmt.Errorf("color_hex must be a valid hex color (e.g., #FF0000)")
+	}
+	if s.InitialGrams <= 0 {
+		return errors.New("initial_grams must be greater than 0")
+	}
+	if s.RemainingGrams < 0 {
+		return errors.New("remaining_grams must be >= 0")
+	}
+	return nil
+}
+
+// ValidateCreateFilamentRequest validates a creation DTO.
+func ValidateCreateFilamentRequest(req dtos.CreateFilamentRequest) error {
+	if req.Name == "" {
+		return errors.New("name is required")
+	}
+	if req.MaterialBaseID <= 0 {
+		return errors.New("material_base_id is required")
+	}
+	if req.MaterialFinishID <= 0 {
+		return errors.New("material_finish_id is required")
+	}
+	if !colorHexPattern.MatchString(req.ColorHex) {
+		return fmt.Errorf("color_hex must be a valid hex color (e.g., #FF0000)")
+	}
+	if req.InitialGrams <= 0 {
+		return errors.New("initial_grams must be greater than 0")
+	}
+	if req.RemainingGrams < 0 {
+		return errors.New("remaining_grams must be >= 0")
+	}
+	return nil
+}
+
+// ValidateUpdateFilamentRequest validates partial update fields.
+func ValidateUpdateFilamentRequest(req dtos.UpdateFilamentRequest) error {
+	if req.ColorHex != nil && !colorHexPattern.MatchString(*req.ColorHex) {
+		return fmt.Errorf("color_hex must be a valid hex color (e.g., #FF0000)")
+	}
+	if req.InitialGrams != nil && *req.InitialGrams <= 0 {
+		return errors.New("initial_grams must be greater than 0")
+	}
+	if req.RemainingGrams != nil && *req.RemainingGrams < 0 {
+		return errors.New("remaining_grams must be >= 0")
+	}
+	return nil
+}