feat(export): CSV export for bookings and customers

GET /v1/export/bookings.csv (optional from/to bounds) and
/v1/export/customers.csv — UTF-8 with BOM for Excel, labels resolved
at export time, buffered responses so auth failures stay JSON.
Export buttons on the bookings and customers pages.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/export/service.go

@@ -0,0 +1,174 @@
+package export
+
+import (
+	"context"
+	"encoding/csv"
+	"errors"
+	"io"
+	"time"
+
+	"bookra/apps/backend/internal/db"
+	"bookra/apps/backend/internal/domain"
+)
+
+var ErrTenantMembership = errors.New("tenant membership not found")
+
+// utf8BOM lets Excel detect the encoding for Czech diacritics.
+var utf8BOM = []byte{0xEF, 0xBB, 0xBF}
+
+const customerPageSize = 500
+
+type Service struct {
+	repo db.Repository
+}
+
+func NewService(repo db.Repository) *Service {
+	return &Service{repo: repo}
+}
+
+func (s *Service) tenantFor(ctx context.Context, principal domain.Principal) (db.TenantRecord, error) {
+	membership, err := s.repo.GetTenantMembershipBySubject(ctx, principal.Subject)
+	if err != nil {
+		return db.TenantRecord{}, ErrTenantMembership
+	}
+	return membership.Tenant, nil
+}
+
+// WriteBookingsCSV streams every tenant booking in [from, to) as CSV rows.
+// Service labels resolve to service names for appointments and session titles
+// for classes, matching how the rest of the app labels them.
+func (s *Service) WriteBookingsCSV(ctx context.Context, principal domain.Principal, w io.Writer, from, to time.Time) error {
+	tenant, err := s.tenantFor(ctx, principal)
+	if err != nil {
+		return err
+	}
+
+	bookings, err := s.repo.ListBookingsByTenantBetween(ctx, tenant.ID, from, to)
+	if err != nil {
+		return err
+	}
+	services, err := s.repo.ListServicesByTenant(ctx, tenant.ID)
+	if err != nil {
+		return err
+	}
+	sessions, err := s.repo.ListClassSessionsByTenant(ctx, tenant.ID, from, 10000)
+	if err != nil {
+		return err
+	}
+	staff, err := s.repo.ListStaffByTenant(ctx, tenant.ID)
+	if err != nil {
+		return err
+	}
+	locations, err := s.repo.ListLocationsByTenant(ctx, tenant.ID)
+	if err != nil {
+		return err
+	}
+
+	serviceNames := make(map[string]string, len(services))
+	for _, service := range services {
+		serviceNames[service.ID] = service.Name
+	}
+	sessionTitles := make(map[string]string, len(sessions))
+	for _, session := range sessions {
+		sessionTitles[session.ID] = session.Title
+	}
+	staffNames := make(map[string]string, len(staff))
+	for _, member := range staff {
+		staffNames[member.ID] = member.DisplayName
+	}
+	locationNames := make(map[string]string, len(locations))
+	for _, location := range locations {
+		locationNames[location.ID] = location.Name
+	}
+
+	if _, err := w.Write(utf8BOM); err != nil {
+		return err
+	}
+	out := csv.NewWriter(w)
+	defer out.Flush()
+
+	if err := out.Write([]string{
+		"reference", "status", "customer_name", "customer_email", "customer_phone",
+		"service", "staff", "location", "starts_at", "ends_at", "created_at", "notes",
+	}); err != nil {
+		return err
+	}
+	for _, booking := range bookings {
+		label := ""
+		if booking.ServiceID != nil {
+			label = serviceNames[*booking.ServiceID]
+		}
+		if booking.ClassSessionID != nil {
+			label = sessionTitles[*booking.ClassSessionID]
+		}
+		staffName := ""
+		if booking.StaffID != nil {
+			staffName = staffNames[*booking.StaffID]
+		}
+		locationName := ""
+		if booking.LocationID != nil {
+			locationName = locationNames[*booking.LocationID]
+		}
+		if err := out.Write([]string{
+			booking.Reference,
+			booking.Status,
+			booking.CustomerName,
+			booking.CustomerEmail,
+			booking.CustomerPhone,
+			label,
+			staffName,
+			locationName,
+			booking.StartsAt.UTC().Format(time.RFC3339),
+			booking.EndsAt.UTC().Format(time.RFC3339),
+			booking.CreatedAt.UTC().Format(time.RFC3339),
+			booking.Notes,
+		}); err != nil {
+			return err
+		}
+	}
+	return out.Error()
+}
+
+// WriteCustomersCSV streams the tenant's full customer list, paging through
+// the repository in bounded chunks.
+func (s *Service) WriteCustomersCSV(ctx context.Context, principal domain.Principal, w io.Writer) error {
+	tenant, err := s.tenantFor(ctx, principal)
+	if err != nil {
+		return err
+	}
+
+	if _, err := w.Write(utf8BOM); err != nil {
+		return err
+	}
+	out := csv.NewWriter(w)
+	defer out.Flush()
+
+	if err := out.Write([]string{"name", "email", "phone", "status", "created_at"}); err != nil {
+		return err
+	}
+	for offset := 0; ; offset += customerPageSize {
+		page, err := s.repo.ListCustomersByTenant(ctx, tenant.ID, customerPageSize, offset)
+		if err != nil {
+			return err
+		}
+		for _, customer := range page {
+			phone := ""
+			if customer.Phone != nil {
+				phone = *customer.Phone
+			}
+			if err := out.Write([]string{
+				customer.Name,
+				customer.Email,
+				phone,
+				customer.Status,
+				customer.CreatedAt.UTC().Format(time.RFC3339),
+			}); err != nil {
+				return err
+			}
+		}
+		if len(page) < customerPageSize {
+			break
+		}
+	}
+	return out.Error()
+}