Reorganize repo: backend/, deploy/, docs/ layout + AGENTS.md

Restructure the monorepo into clear top-level directories:
- backend/: Go module root (cmd, internal, pkg, configs, migrations,
  docs/swagger, scripts, tests, go.mod, Makefile, .air.toml)
- deploy/: Docker (Dockerfile, docker-compose*), quickstart, fluentd
- docs/: project documentation + reports/ (moved from repo root)
- AGENTS.md: new AI coding-agent guide at repo root

Update all references to the new layout:
- Dockerfile: COPY backend/go.mod, COPY backend/ (context = repo root)
- docker-compose files: context ../.., dockerfile deploy/docker/Dockerfile,
  env_file ../../.env, volume mounts ../../backend:/app
- deploy/quickstart/compose.yaml: dockerfile deploy/docker/Dockerfile
- CI: working-directory: backend for go commands, file deploy/docker/Dockerfile,
  coverage path backend/coverage.out, health_check backend/scripts/
- backend/Makefile: docker target uses -f ../deploy/docker/Dockerfile ../
- README: architecture tree, quickstart, config paths updated

Move root stray scripts (rename_models.*, run_m11_tests.sh, verify_build.sh,
gorm_bool_main.go) to backend/scripts/legacy/. All moves via git mv to
preserve history. Build, vet, SQLite tests, and docker compose config verified.

backend/internal/handler/api/v1/notification_setting_handler.go

@@ -0,0 +1,105 @@
+package v1
+
+// NotificationSettingHandler handles notification setting API endpoints.
+// Reference: Chatwoot NotificationSettingsController — show + update
+//
+// Routes:
+//   GET    /api/v1/accounts/:account_id/notification_settings
+//   PATCH  /api/v1/accounts/:account_id/notification_settings
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/gochat/gochat/internal/model"
+	"github.com/gochat/gochat/internal/service"
+	"github.com/gochat/gochat/pkg/response"
+)
+
+// NotificationSettingHandler handles HTTP requests for notification settings.
+type NotificationSettingHandler struct {
+	svc *service.NotificationSettingService
+}
+
+// NewNotificationSettingHandler creates a new handler instance.
+func NewNotificationSettingHandler(svc *service.NotificationSettingService) *NotificationSettingHandler {
+	return &NotificationSettingHandler{svc: svc}
+}
+
+// NotificationSettingUpdateWrapper matches Chatwoot params.require(:notification_settings) —
+// request body must be nested under "notification_settings" key.
+type NotificationSettingUpdateWrapper struct {
+	NotificationSettings service.UpdateNotificationSettingRequest `json:"notification_settings"`
+}
+
+// Show returns the notification setting for the current user in the account.
+// GET /api/v1/accounts/:account_id/notification_settings
+// Reference: Chatwoot show — @user.notification_settings.find_by(account_id: Current.account.id)
+// Jbuilder: id, user_id, account_id, all_email_flags, selected_email_flags, all_push_flags, selected_push_flags
+func (h *NotificationSettingHandler) Show(c *gin.Context) {
+	accountID := getAccountID(c)
+	if accountID == 0 {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid account_id")
+		return
+	}
+
+	userID := getUserID(c)
+	if userID == 0 {
+		response.AbortWithStatusError(c, http.StatusUnauthorized, response.ErrUnauthorized, "user not authenticated")
+		return
+	}
+
+	ns, svcErr := h.svc.Get(c.Request.Context(), accountID, userID)
+	if svcErr != nil {
+		handleServiceError(c, svcErr)
+		return
+	}
+
+	c.JSON(http.StatusOK, serializeNotificationSetting(ns))
+}
+
+// Update modifies notification settings for the current user in the account.
+// PATCH /api/v1/accounts/:account_id/notification_settings
+// Reference: Chatwoot update — params.require(:notification_settings).permit(selected_email_flags: [], selected_push_flags: [])
+// Then update_flags → save → render show
+func (h *NotificationSettingHandler) Update(c *gin.Context) {
+	accountID := getAccountID(c)
+	if accountID == 0 {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid account_id")
+		return
+	}
+
+	userID := getUserID(c)
+	if userID == 0 {
+		response.AbortWithStatusError(c, http.StatusUnauthorized, response.ErrUnauthorized, "user not authenticated")
+		return
+	}
+
+	var wrapper NotificationSettingUpdateWrapper
+	if err := c.ShouldBindJSON(&wrapper); err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, response.ErrBadRequest, "invalid request body")
+		return
+	}
+
+	ns, svcErr := h.svc.Update(c.Request.Context(), accountID, userID, wrapper.NotificationSettings)
+	if svcErr != nil {
+		handleServiceError(c, svcErr)
+		return
+	}
+
+	c.JSON(http.StatusOK, serializeNotificationSetting(ns))
+}
+
+// serializeNotificationSetting builds the response object matching Chatwoot Jbuilder show.json.jbuilder:
+// id, user_id, account_id, all_email_flags, selected_email_flags, all_push_flags, selected_push_flags
+func serializeNotificationSetting(ns *model.NotificationSetting) gin.H {
+	return gin.H{
+		"id":                   ns.ID,
+		"user_id":              ns.UserID,
+		"account_id":           ns.AccountID,
+		"all_email_flags":      model.AllEmailFlagNames(),
+		"selected_email_flags": ns.SelectedEmailFlagNames(),
+		"all_push_flags":       model.AllPushFlagNames(),
+		"selected_push_flags":  ns.SelectedPushFlagNames(),
+	}
+}