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/pagination/pagination.go

@@ -0,0 +1,50 @@
+package pagination
+
+import (
+	"strconv"
+
+	"github.com/gin-gonic/gin"
+)
+
+// DefaultPerPage is the default page size.
+// Reference: Chatwoot API default pagination (25 items per page)
+const DefaultPerPage = 25
+const MaxPerPage = 100
+
+// Params holds pagination parameters extracted from request
+type Params struct {
+	Page    int
+	PerPage int
+	Offset  int
+}
+
+// Parse extracts pagination params from Gin context query.
+// Pattern: Chatwoot controllers use params[:page] and params[:per_page]
+func Parse(c *gin.Context) Params {
+	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
+	perPage, _ := strconv.Atoi(c.DefaultQuery("per_page", strconv.Itoa(DefaultPerPage)))
+
+	if page < 1 {
+		page = 1
+	}
+	if perPage < 1 {
+		perPage = DefaultPerPage
+	}
+	if perPage > MaxPerPage {
+		perPage = MaxPerPage
+	}
+
+	return Params{
+		Page:    page,
+		PerPage: perPage,
+		Offset:  (page - 1) * perPage,
+	}
+}
+
+// OffsetToPage converts offset back to page number
+func OffsetToPage(offset, perPage int) int {
+	if perPage <= 0 {
+		return 1
+	}
+	return (offset / perPage) + 1
+}