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/repositories/printer_repository.go

@@ -0,0 +1,78 @@
+package repositories
+
+import (
+	"context"
+
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/models"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// PrinterRepository handles database queries for printers.
+type PrinterRepository struct {
+	pool *pgxpool.Pool
+}
+
+// NewPrinterRepository creates a PrinterRepository backed by the given pool.
+func NewPrinterRepository(pool *pgxpool.Pool) *PrinterRepository {
+	return &PrinterRepository{pool: pool}
+}
+
+// scanPrinterWithType scans a printer row with JOINed printer_type.
+func (r *PrinterRepository) scanPrinterWithType(row interface{ Scan(...interface{}) error }) (models.Printer, error) {
+	var p models.Printer
+	var pt models.PrinterType
+
+	err := row.Scan(
+		&p.ID, &p.Name, &p.PrinterTypeID,
+		&p.Manufacturer, &p.Model,
+		&p.MoonrakerURL, &p.MoonrakerAPIKey,
+		&p.MQTTBrokerHost, &p.MQTTTopicPrefix,
+		&p.MQTTTLSEnabled, &p.IsActive,
+		&p.CreatedAt, &p.UpdatedAt,
+		&pt.ID, &pt.Name,
+		&pt.CreatedAt, &pt.UpdatedAt,
+	)
+	if err != nil {
+		return p, err
+	}
+
+	p.PrinterType = &pt
+	return p, nil
+}
+
+// GetAll returns all printers joined with their printer_type, ordered by name.
+func (r *PrinterRepository) GetAll(ctx context.Context) ([]models.Printer, error) {
+	rows, err := r.pool.Query(ctx, `
+		SELECT p.id, p.name, p.printer_type_id,
+		       p.manufacturer, p.model,
+		       p.moonraker_url, p.moonraker_api_key,
+		       p.mqtt_broker_host, p.mqtt_topic_prefix,
+		       p.mqtt_tls_enabled, p.is_active,
+		       p.created_at, p.updated_at,
+		       pt.id, pt.name,
+		       pt.created_at, pt.updated_at
+		FROM printers p
+		JOIN printer_types pt ON p.printer_type_id = pt.id
+		ORDER BY p.name
+	`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var printers []models.Printer
+	for rows.Next() {
+		p, err := r.scanPrinterWithType(rows)
+		if err != nil {
+			return nil, err
+		}
+		printers = append(printers, p)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	if printers == nil {
+		printers = []models.Printer{}
+	}
+	return printers, nil
+}