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/push_token.go

@@ -0,0 +1,23 @@
+package model
+
+import (
+	"time"
+)
+
+// PushToken represents a browser/mobile push notification token.
+// Reference: Chatwoot PushSubscription model + P2B M8 spec
+type PushToken struct {
+	ID            uint      `gorm:"primaryKey" json:"id"`
+	UserID        uint      `gorm:"not null;index" json:"user_id"`
+	Token         string    `gorm:"size:512;not null;uniqueIndex" json:"token"` // endpoint URL for web, device token for mobile
+	Platform      string    `gorm:"size:50" json:"platform"`                    // ios/android/web
+	DeviceID      string    `gorm:"size:255" json:"device_id"`
+	P256DHKey     string    `gorm:"size:256" json:"p256dh_key,omitempty"` // Web Push: client public key (ECDH)
+	AuthKey       string    `gorm:"size:64" json:"auth_key,omitempty"`    // Web Push: client auth secret
+	CreatedAt     time.Time `gorm:"autoCreateTime" json:"created_at"`
+	UpdatedAt     time.Time `gorm:"autoUpdateTime" json:"updated_at"`
+
+	User User `gorm:"foreignKey:UserID" json:"user,omitempty"`
+}
+
+func (PushToken) TableName() string { return "push_tokens" }