refactor(api): extract repository layer and thin HTTP handlers

Introduce internal/repository for data access and cache-backed system
config reads. Move business logic into logics.go across admin push,
user, template, cache, system_config, and upload/handler packages.

Remove Gin from internal/util by relocating request-scoped helpers to
oauth/gin_context.go. Propagate request context for config lookups in
user flows. Slim model entities and delete model-level DB/cache helpers.

Wire handlers to logics/repository so targeted packages no longer call
db.DB directly. Update admin router tests to use ErrorHandlerMiddleware.

internal/apps/oauth/session_context.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/Rain-kl/Wavelet/internal/config"
 	"github.com/Rain-kl/Wavelet/internal/model"
+	"github.com/Rain-kl/Wavelet/internal/repository"
 	"github.com/gin-contrib/sessions"
 	"github.com/gin-gonic/gin"
 	"github.com/google/uuid"
@@ -56,7 +57,7 @@ func setLoginSession(ctx context.Context, c *gin.Context, user *model.User) erro
 	maxAge := config.Config.App.SessionAge
 	isSessionCookie := false
 
-	ttlHours, err := model.GetIntByKey(ctx, model.ConfigKeyLoginSessionTTLHours)
+	ttlHours, err := repository.GetIntByKey(ctx, model.ConfigKeyLoginSessionTTLHours)
 	if err == nil {
 		switch {
 		case ttlHours == -1: