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/webhookutil/timeout.go

@@ -0,0 +1,31 @@
+package webhookutil
+
+import (
+	"context"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/gochat/gochat/internal/model"
+	"gorm.io/gorm"
+)
+
+const (
+	TimeoutConfigName = "WEBHOOK_TIMEOUT"
+	DefaultTimeout    = 5 * time.Second
+)
+
+func Timeout(ctx context.Context, db *gorm.DB) time.Duration {
+	if db == nil {
+		return DefaultTimeout
+	}
+	var cfg model.InstallationConfig
+	if err := db.WithContext(ctx).Where("name = ?", TimeoutConfigName).First(&cfg).Error; err != nil {
+		return DefaultTimeout
+	}
+	seconds, err := strconv.Atoi(strings.TrimSpace(cfg.Value))
+	if err != nil || seconds <= 0 {
+		return DefaultTimeout
+	}
+	return time.Duration(seconds) * time.Second
+}