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/widget_offline_message.go

@@ -0,0 +1,55 @@
+package model
+
+import "time"
+
+// WidgetOfflineMessage stores messages submitted by visitors when no agents are online.
+// Reference: Chatwoot app/models/channel/web_widget.rb — agent_away_message + offline messaging
+//
+// When a visitor sends a message via the widget and no agents are available
+// (based on business hours or agent presence), the message is stored as an offline
+// message. When an agent becomes available, the offline message is converted into
+// a conversation automatically.
+//
+// This replaces the simpler approach of just showing an away message — visitors
+// can still leave their contact info and message, which creates a pending conversation
+// that agents can respond to when they return.
+type WidgetOfflineMessage struct {
+	Base
+	InboxID        uint          `gorm:"index;not null" json:"inbox_id"`                        // The web_widget inbox
+	AccountID      uint          `gorm:"index;not null" json:"account_id"`                      // Account scope for the message
+	ContactName    string        `gorm:"size:255" json:"contact_name,omitempty"`                // Visitor name (from pre-chat form or anonymous)
+	ContactEmail   string        `gorm:"size:512" json:"contact_email,omitempty"`               // Visitor email (from pre-chat form)
+	ContactPhone   string        `gorm:"size:30" json:"contact_phone,omitempty"`                // Visitor phone (from pre-chat form)
+	ContactCompany string        `gorm:"size:255" json:"contact_company,omitempty"`             // Visitor company (from pre-chat form)
+	ContactCity    string        `gorm:"size:255" json:"contact_city,omitempty"`                // Visitor city (from pre-chat form)
+	ContactCountry string        `gorm:"size:255" json:"contact_country,omitempty"`             // Visitor country (from pre-chat form)
+	Content        string        `gorm:"type:text;not null" json:"content"`                     // The message content
+	Referer        string        `gorm:"size:1024" json:"referer,omitempty"`                    // Page URL where widget was embedded
+	BrowserInfo    string        `gorm:"size:512" json:"browser_info,omitempty"`                // Browser metadata
+	ConversationID *uint         `gorm:"index" json:"conversation_id,omitempty"`                // Set when converted to a conversation (null until then)
+	Status         OfflineStatus `gorm:"size:30;default:'pending'" json:"status"`               // pending → converted → dismissed
+	ConvertedAt    *time.Time    `json:"converted_at,omitempty"`                                 // When the message was converted to a conversation
+}
+
+func (WidgetOfflineMessage) TableName() string { return "widget_offline_messages" }
+
+// OfflineStatus represents the lifecycle of an offline message.
+type OfflineStatus string
+
+const (
+	OfflineStatusPending   OfflineStatus = "pending"   // Awaiting agent to come online
+	OfflineStatusConverted OfflineStatus = "converted"  // Converted into a conversation
+	OfflineStatusDismissed OfflineStatus = "dismissed"  // Dismissed by admin without conversion
+)
+
+// WidgetOfflineMessageSubmission is the request body from the widget SDK
+// when a visitor submits a message while agents are offline.
+type WidgetOfflineMessageSubmission struct {
+	Name    string `json:"name,omitempty"`              // Visitor name
+	Email   string `json:"email,omitempty"`             // Visitor email
+	Phone   string `json:"phone,omitempty"`             // Visitor phone
+	Company string `json:"company,omitempty"`           // Visitor company
+	City    string `json:"city,omitempty"`              // Visitor city
+	Country string `json:"country,omitempty"`           // Visitor country
+	Message string `json:"message" binding:"required"`  // The offline message content
+}