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/model/common.go

@@ -0,0 +1,18 @@
+package model
+
+// ErrorResponse represents a standard API error response for Swagger documentation.
+type ErrorResponse struct {
+	Error   string `json:"error" example:"not_found"`
+	Message string `json:"message" example:"Resource not found"`
+}
+
+// PaginatedResponse represents a paginated list response for Swagger documentation.
+type PaginatedResponse struct {
+	Data interface{} `json:"data"`
+	Meta struct {
+		CurrentPage int `json:"current_page" example:"1"`
+		PerPage    int `json:"per_page" example:"25"`
+		TotalPages int `json:"total_pages" example:"10"`
+		TotalCount int `json:"total_count" example:"250"`
+	} `json:"meta"`
+}