refactor(structure): group platform, infra, and shared packages

Reorganize internal packages into platform/infra/shared layers and update
imports, docs, and seed-count tests to match current system configs.

internal/testhelper/gin.go

@@ -4,7 +4,7 @@
 package testhelper
 
 import (
-	"github.com/Rain-kl/Wavelet/internal/common/response"
+	"github.com/Rain-kl/Wavelet/internal/shared/response"
 	"github.com/gin-gonic/gin"
 )
 

internal/testhelper/test_helper.go

@@ -9,7 +9,7 @@ import (
 	"context"
 	"testing"
 
-	"github.com/Rain-kl/Wavelet/internal/db"
+	db "github.com/Rain-kl/Wavelet/internal/infra/persistence"
 	"github.com/Rain-kl/Wavelet/internal/model"
 	"github.com/Rain-kl/Wavelet/internal/repository"
 	"github.com/alicebob/miniredis/v2"