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/notification.go

@@ -0,0 +1,33 @@
+package model
+
+import (
+	"encoding/json"
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// Notification represents a notification delivered to a user.
+// Reference: Chatwoot Notification model + P2B M8 spec
+type Notification struct {
+	ID                   uint           `gorm:"primaryKey" json:"id"`
+	AccountID            *uint          `gorm:"index" json:"account_id,omitempty"`
+	UserID               uint           `gorm:"not null;index" json:"user_id"`
+	NotificationType     string         `gorm:"size:100;not null;index" json:"notification_type"` // conversation_created/conversation_assigned/message_created/assigned_conversation_new_message
+	PrimaryActorType     string         `gorm:"size:100" json:"primary_actor_type"`  // Conversation/Message/User/Contact
+	PrimaryActorID       uint           `json:"primary_actor_id"`
+	SecondaryActorType   string         `gorm:"size:100" json:"secondary_actor_type"`
+	SecondaryActorID     uint           `json:"secondary_actor_id"`
+	ReadAt               *time.Time     `json:"read_at,omitempty"`
+	SnoozedUntil         *time.Time     `json:"snoozed_until,omitempty"`
+	PushEnabled          bool           `gorm:"default:false" json:"push_enabled"`
+	EmailEnabled         bool           `gorm:"default:false" json:"email_enabled"`
+	AdditionalAttributes json.RawMessage `gorm:"type:jsonb" json:"additional_attributes"`
+	CreatedAt            time.Time      `gorm:"autoCreateTime;index" json:"created_at"`
+	UpdatedAt            time.Time      `gorm:"autoUpdateTime" json:"updated_at"`
+	DeletedAt            gorm.DeletedAt `gorm:"index" json:"deleted_at,omitempty"`
+
+	User User `gorm:"foreignKey:UserID" json:"user,omitempty"`
+}
+
+func (Notification) TableName() string { return "notifications" }