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/api/server.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -21,6 +22,7 @@ import (
 	"bookra/apps/backend/internal/config"
 	"bookra/apps/backend/internal/db"
 	"bookra/apps/backend/internal/domain"
+	"bookra/apps/backend/internal/export"
 	"bookra/apps/backend/internal/httpx"
 	"bookra/apps/backend/internal/notifications"
 	"bookra/apps/backend/internal/sms"
@@ -59,6 +61,7 @@ func NewServer(cfg config.Config, pools *db.Pools) (*Server, error) {
 	catalogService := catalog.NewService(repository, billingService, notificationService)
 	calendarService := calendar.NewService(repository)
 	activityService := activity.NewService(repository)
+	exportService := export.NewService(repository)
 	authService := auth.NewService(repository, cfg.AuthJWTSecret)
 	adminService := admin.NewService(repository, cfg.AdminEmail, cfg.AdminKey)
 	smsService := sms.NewService(cfg, repository)
@@ -1293,6 +1296,37 @@ func NewServer(cfg config.Config, pools *db.Pools) (*Server, error) {
 		c.JSON(http.StatusOK, response)
 	})
 
+	protected.GET("/export/bookings.csv", func(c *gin.Context) {
+		from, to := exportRange(c)
+		var buf bytes.Buffer
+		err := exportService.WriteBookingsCSV(c.Request.Context(), auth.PrincipalFromContext(c), &buf, from, to)
+		if err != nil {
+			status := http.StatusInternalServerError
+			if errors.Is(err, export.ErrTenantMembership) {
+				status = http.StatusNotFound
+			}
+			c.JSON(status, gin.H{"error": err.Error()})
+			return
+		}
+		c.Header("Content-Disposition", `attachment; filename="bookra-bookings.csv"`)
+		c.Data(http.StatusOK, "text/csv; charset=utf-8", buf.Bytes())
+	})
+
+	protected.GET("/export/customers.csv", func(c *gin.Context) {
+		var buf bytes.Buffer
+		err := exportService.WriteCustomersCSV(c.Request.Context(), auth.PrincipalFromContext(c), &buf)
+		if err != nil {
+			status := http.StatusInternalServerError
+			if errors.Is(err, export.ErrTenantMembership) {
+				status = http.StatusNotFound
+			}
+			c.JSON(status, gin.H{"error": err.Error()})
+			return
+		}
+		c.Header("Content-Disposition", `attachment; filename="bookra-customers.csv"`)
+		c.Data(http.StatusOK, "text/csv; charset=utf-8", buf.Bytes())
+	})
+
 	protected.GET("/billing/subscription", func(c *gin.Context) {
 		response, err := billingService.GetSubscription(c.Request.Context(), auth.PrincipalFromContext(c))
 		if err != nil {
@@ -1578,6 +1612,24 @@ func (s *Server) runTrialEndingCheck() {
 	}
 }
 
+// exportRange parses optional from/to=YYYY-MM-DD bounds for CSV export;
+// the default is effectively all-time.
+func exportRange(c *gin.Context) (time.Time, time.Time) {
+	from := time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC)
+	to := time.Date(2100, 1, 1, 0, 0, 0, 0, time.UTC)
+	if value := c.Query("from"); value != "" {
+		if parsed, err := time.Parse("2006-01-02", value); err == nil {
+			from = parsed
+		}
+	}
+	if value := c.Query("to"); value != "" {
+		if parsed, err := time.Parse("2006-01-02", value); err == nil {
+			to = parsed.AddDate(0, 0, 1) // inclusive day bound
+		}
+	}
+	return from, to
+}
+
 func authorizeJobRunner(c *gin.Context, cfg config.Config) bool {
 	if cfg.JobRunnerKey == "" {
 		return false