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/message.go

@@ -0,0 +1,31 @@
+package model
+
+import (
+	"gorm.io/datatypes"
+)
+
+// Message represents a message within a conversation.
+type Message struct {
+	Base
+	ConversationID       uint           `gorm:"index;not null" json:"conversation_id"`
+	AccountID            uint           `gorm:"index;not null" json:"account_id"`
+	InboxID              uint           `gorm:"index;not null" json:"inbox_id"`
+	SenderID             *uint          `gorm:"index" json:"sender_id,omitempty"`
+	SenderType           string         `gorm:"size:50" json:"sender_type"` // contact, agent, bot
+	Content              string         `gorm:"type:text" json:"content"`
+	ContentType          string         `gorm:"size:50;default:text" json:"content_type"` // text, input, input_csat, file, image, etc
+	Status               string         `gorm:"size:50;default:sent" json:"status"`       // sent, delivered, read, failed
+	Private              bool           `gorm:"default:false" json:"private"`
+	EchoID               string         `gorm:"size:255" json:"echo_id,omitempty"`
+	SourceID             string         `gorm:"size:255" json:"source_id,omitempty"`
+	MessageType          string         `gorm:"size:50;default:incoming" json:"message_type"` // incoming, outgoing, activity, template
+	External             bool           `gorm:"default:false" json:"external"`
+	ContentAttributes    datatypes.JSON `gorm:"type:jsonb" json:"content_attributes,omitempty"` // attachments, mentions, etc
+	AdditionalAttributes datatypes.JSON `gorm:"type:jsonb" json:"additional_attributes,omitempty"`
+	ExternalSourceIDs    datatypes.JSON `gorm:"type:jsonb" json:"external_source_ids,omitempty"` // external platform IDs
+
+	Conversation *Conversation `gorm:"foreignKey:ConversationID" json:"conversation,omitempty"`
+	Attachments  []Attachment  `gorm:"foreignKey:MessageID" json:"attachments,omitempty"`
+}
+
+func (Message) TableName() string { return "messages" }