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/audit_writer.go

@@ -0,0 +1,46 @@
+package v1
+
+import (
+	"github.com/gin-gonic/gin"
+
+	"github.com/gochat/gochat/internal/service"
+	applogger "github.com/gochat/gochat/pkg/logger"
+)
+
+type auditMutation struct {
+	AccountID      uint
+	AuditableType  string
+	AuditableID    uint
+	Action         string
+	AuditedChanges interface{}
+	Comment        string
+}
+
+func recordAuditMutation(c *gin.Context, auditSvc *service.AuditService, mutation auditMutation) {
+	if auditSvc == nil {
+		return
+	}
+	_, err := auditSvc.Record(c.Request.Context(), service.AuditRecord{
+		AccountID:      mutation.AccountID,
+		UserID:         getUserID(c),
+		AuditableType:  mutation.AuditableType,
+		AuditableID:    mutation.AuditableID,
+		Action:         mutation.Action,
+		AuditedChanges: mutation.AuditedChanges,
+		RemoteAddress:  c.ClientIP(),
+		RequestUUID:    firstNonEmpty(c.GetHeader("X-Request-ID"), c.GetHeader("X-Correlation-ID")),
+		Comment:        mutation.Comment,
+	})
+	if err != nil {
+		applogger.L().Warnf("audit mutation skipped type=%s id=%d action=%s: %v", mutation.AuditableType, mutation.AuditableID, mutation.Action, err)
+	}
+}
+
+func firstNonEmpty(values ...string) string {
+	for _, value := range values {
+		if value != "" {
+			return value
+		}
+	}
+	return ""
+}