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/inbox_handler_test.go_BAK

@@ -0,0 +1,115 @@
+package v1
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+
+	"github.com/gochat/gochat/internal/service"
+)
+
+func setupInboxRouter(handler *InboxHandler) *gin.Engine {
+	gin.SetMode(gin.TestMode)
+	router := gin.New()
+	router.GET("/api/v1/accounts/:id/inboxes", handler.List)
+	router.GET("/api/v1/accounts/:id/inboxes/:inbox_id", handler.Get)
+	router.POST("/api/v1/accounts/:id/inboxes", handler.Create)
+	router.PUT("/api/v1/accounts/:id/inboxes/:inbox_id", handler.Update)
+	router.DELETE("/api/v1/accounts/:id/inboxes/:inbox_id", handler.Delete)
+	return router
+}
+
+func TestInboxListBadAccountID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("GET", "/api/v1/accounts/abc/inboxes", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+	var resp map[string]interface{}
+	json.Unmarshal(w.Body.Bytes(), &resp)
+	assert.Contains(t, resp, "error")
+}
+
+func TestInboxGetBadAccountID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("GET", "/api/v1/accounts/abc/inboxes/5", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+}
+
+func TestInboxGetBadInboxID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("GET", "/api/v1/accounts/1/inboxes/abc", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+}
+
+func TestInboxCreateBadAccountID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("POST", "/api/v1/accounts/abc/inboxes", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+}
+
+func TestInboxUpdateBadAccountID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("PUT", "/api/v1/accounts/abc/inboxes/5", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+}
+
+func TestInboxUpdateBadInboxID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("PUT", "/api/v1/accounts/1/inboxes/abc", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+}
+
+func TestInboxDeleteBadAccountID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("DELETE", "/api/v1/accounts/abc/inboxes/5", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+}
+
+func TestInboxDeleteBadInboxID(t *testing.T) {
+	handler := NewInboxHandler(&service.InboxService{})
+	router := setupInboxRouter(handler)
+
+	w := httptest.NewRecorder()
+	req, _ := http.NewRequest("DELETE", "/api/v1/accounts/1/inboxes/abc", nil)
+	router.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+}