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/dtos/dtos.go

@@ -0,0 +1,67 @@
+// Package dtos defines request/response data transfer objects for the Extrudex API.
+// DTOs keep HTTP serialization concerns separate from domain models.
+package dtos
+
+// ============================================================================
+// Common Response Wrappers
+// ============================================================================
+
+// ListResponse wraps a paginated collection response.
+type ListResponse struct {
+	Data   any `json:"data"`
+	Total  int `json:"total"`
+	Limit  int `json:"limit"`
+	Offset int `json:"offset"`
+}
+
+// SingleResponse wraps a single-item response.
+type SingleResponse struct {
+	Data any `json:"data"`
+}
+
+// ErrorResponse is the standard error payload for all API errors.
+type ErrorResponse struct {
+	Error string `json:"error"`
+	Code  int    `json:"code"`
+}
+
+// ============================================================================
+// Filament DTOs
+// ============================================================================
+
+// CreateFilamentRequest is the POST body for creating a new filament spool.
+type CreateFilamentRequest struct {
+	Name                  string  `json:"name"`
+	MaterialBaseID        int     `json:"material_base_id"`
+	MaterialFinishID      int     `json:"material_finish_id"`
+	MaterialModifierID    *int    `json:"material_modifier_id,omitempty"`
+	ColorHex             string  `json:"color_hex"`
+	Brand                *string `json:"brand,omitempty"`
+	DiameterMM           *float64 `json:"diameter_mm,omitempty"` // defaults to 1.75
+	InitialGrams         int     `json:"initial_grams"`
+	RemainingGrams       int     `json:"remaining_grams"`
+	SpoolWeightGrams     *int    `json:"spool_weight_grams,omitempty"`
+	CostUSD              *float64 `json:"cost_usd,omitempty"`
+	LowStockThresholdGrams *int  `json:"low_stock_threshold_grams,omitempty"` // defaults to 50
+	Notes                *string `json:"notes,omitempty"`
+	Barcode              *string `json:"barcode,omitempty"`
+}
+
+// UpdateFilamentRequest is the PUT body for partially updating a filament spool.
+// All fields are optional — only non-nil fields are applied.
+type UpdateFilamentRequest struct {
+	Name                  *string  `json:"name,omitempty"`
+	MaterialBaseID        *int     `json:"material_base_id,omitempty"`
+	MaterialFinishID      *int     `json:"material_finish_id,omitempty"`
+	MaterialModifierID    *int     `json:"material_modifier_id,omitempty"`
+	ColorHex             *string  `json:"color_hex,omitempty"`
+	Brand                *string  `json:"brand,omitempty"`
+	DiameterMM           *float64 `json:"diameter_mm,omitempty"`
+	InitialGrams         *int     `json:"initial_grams,omitempty"`
+	RemainingGrams       *int     `json:"remaining_grams,omitempty"`
+	SpoolWeightGrams     *int     `json:"spool_weight_grams,omitempty"`
+	CostUSD              *float64 `json:"cost_usd,omitempty"`
+	LowStockThresholdGrams *int   `json:"low_stock_threshold_grams,omitempty"`
+	Notes                *string  `json:"notes,omitempty"`
+	Barcode              *string  `json:"barcode,omitempty"`
+}