feat(sms): implement SMS messaging and metered billing

Implement a complete SMS messaging system including:
- Integration with SMS Manager.cz API for sending messages.
- Metered billing via Stripe using monthly aggregate invoice items.
- Backend services for managing SMS settings, usage logging, and monthly reporting.
- Database migrations for tenant settings, usage logs, and monthly reports.
- Frontend dashboard components for SMS configuration, usage tracking, and history.
- Support for customer phone numbers in the booking flow.

Includes new migrations, backend services, and frontend UI components.

apps/backend/internal/sms/billing.go

@@ -0,0 +1,80 @@
+package sms
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"bookra/apps/backend/internal/config"
+	"bookra/apps/backend/internal/db"
+
+	"github.com/stripe/stripe-go/v81"
+	"github.com/stripe/stripe-go/v81/invoiceitem"
+)
+
+type BillingService struct {
+	cfg         config.Config
+	repo        db.Repository
+	smsPriceIDs map[string]string // currency -> price ID
+}
+
+func NewBillingService(cfg config.Config, repo db.Repository) *BillingService {
+	return &BillingService{
+		cfg:         cfg,
+		repo:        repo,
+		smsPriceIDs: cfg.StripeSMSPriceMatrix,
+	}
+}
+
+// PriceIDForCurrency returns the Stripe price ID for SMS in the given currency
+func (b *BillingService) PriceIDForCurrency(currency string) string {
+	c := strings.ToLower(strings.TrimSpace(currency))
+	if c == "" {
+		c = "czk"
+	}
+	if id := b.smsPriceIDs[c]; id != "" {
+		return id
+	}
+	// Fallback to CZK
+	return b.smsPriceIDs["czk"]
+}
+
+// CreateMonthlyInvoiceItem creates a Stripe InvoiceItem for the total SMS usage of a month.
+// This adds a line item to the customer's next invoice — charging all messages together.
+func (b *BillingService) CreateMonthlyInvoiceItem(ctx context.Context, customerID string, currency string, yearMonth string, messageCount int, totalCents int) (string, error) {
+	if customerID == "" {
+		return "", fmt.Errorf("customer id is empty")
+	}
+
+	priceID := b.PriceIDForCurrency(currency)
+	c := strings.ToLower(strings.TrimSpace(currency))
+	if c == "" {
+		c = "czk"
+	}
+
+	// If a Stripe price is configured, use Price + Quantity for a clean invoice line
+	if priceID != "" {
+		item, err := invoiceitem.New(&stripe.InvoiceItemParams{
+			Customer:    stripe.String(customerID),
+			Price:       stripe.String(priceID),
+			Quantity:    stripe.Int64(int64(messageCount)),
+			Description: stripe.String(fmt.Sprintf("SMS Messages (%s) — %d messages", yearMonth, messageCount)),
+		})
+		if err != nil {
+			return "", fmt.Errorf("failed to create invoice item: %w", err)
+		}
+		return item.ID, nil
+	}
+
+	// Fallback: explicit amount (for dev/testing when no price configured yet)
+	item, err := invoiceitem.New(&stripe.InvoiceItemParams{
+		Customer:    stripe.String(customerID),
+		Amount:      stripe.Int64(int64(totalCents)),
+		Currency:    stripe.String(c),
+		Description: stripe.String(fmt.Sprintf("SMS Messages (%s) — %d messages", yearMonth, messageCount)),
+	})
+	if err != nil {
+		return "", fmt.Errorf("failed to create invoice item: %w", err)
+	}
+	return item.ID, nil
+}