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/pkg/response/response.go

@@ -0,0 +1,85 @@
+package response
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// APIResponse is the unified response structure for all API endpoints.
+// Pattern follows Chatwoot's JSON response format in controllers.
+type APIResponse struct {
+	Success bool        `json:"success"`
+	Data    interface{} `json:"data,omitempty"`
+	Error   *ErrorBody  `json:"error,omitempty"`
+	Meta    *MetaBody   `json:"meta,omitempty"`
+}
+
+type ErrorBody struct {
+	Code    ErrorCode `json:"code"`
+	Message string    `json:"message"`
+	Detail  string    `json:"detail,omitempty"`
+}
+
+type MetaBody struct {
+	Page       int   `json:"page,omitempty"`
+	PerPage    int   `json:"per_page,omitempty"`
+	TotalCount int64 `json:"total_count,omitempty"`
+}
+
+// OK sends a successful response with data
+func OK(c *gin.Context, data interface{}) {
+	c.JSON(http.StatusOK, APIResponse{
+		Success: true,
+		Data:    data,
+	})
+}
+
+// OKWithMeta sends a successful paginated response
+func OKWithMeta(c *gin.Context, data interface{}, page, perPage int, total int64) {
+	c.JSON(http.StatusOK, APIResponse{
+		Success: true,
+		Data:    data,
+		Meta: &MetaBody{
+			Page:       page,
+			PerPage:    perPage,
+			TotalCount: total,
+		},
+	})
+}
+
+// Created sends a 201 response
+func Created(c *gin.Context, data interface{}) {
+	c.JSON(http.StatusCreated, APIResponse{
+		Success: true,
+		Data:    data,
+	})
+}
+
+// NoContent sends a 204 response
+func NoContent(c *gin.Context) {
+	c.Status(http.StatusNoContent)
+}
+
+// AbortWithError sends an error response and aborts the Gin context
+func AbortWithError(c *gin.Context, appErr *AppError) {
+	c.AbortWithStatusJSON(appErr.Status, APIResponse{
+		Success: false,
+		Error: &ErrorBody{
+			Code:    appErr.Code,
+			Message: appErr.Message,
+			Detail:  appErr.Detail,
+		},
+	})
+}
+
+// AbortWithStatusError sends a generic error with just HTTP status and message
+func AbortWithStatusError(c *gin.Context, status int, code ErrorCode, message string) {
+	c.AbortWithStatusJSON(status, APIResponse{
+		Success: false,
+		Error: &ErrorBody{
+			Code:    code,
+			Message: message,
+		},
+	})
+}