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/channel/api.go

@@ -0,0 +1,29 @@
+package channel
+
+import (
+	"time"
+
+	"github.com/gochat/gochat/internal/model"
+	"gorm.io/datatypes"
+)
+
+// ChannelAPI represents a REST API channel configuration.
+// Reference: Chatwoot Channel::Api + P2B M2 spec
+type ChannelAPI struct {
+	ID                   uint           `gorm:"primaryKey" json:"id"`
+	InboxID              uint           `gorm:"uniqueIndex;not null" json:"inbox_id"`
+	WebhookURL           string         `gorm:"size:512" json:"webhook_url"`
+	Secret               string         `gorm:"size:255" json:"secret"`
+	HMACToken            string         `gorm:"size:255" json:"hmac_token"`
+	HMACMandatory        bool           `gorm:"column:hmac_mandatory;default:false" json:"hmac_mandatory"`
+	AdditionalAttributes datatypes.JSON `gorm:"type:jsonb;default:'{}'" json:"additional_attributes"`
+	Identifier           string         `gorm:"size:255" json:"identifier"`
+	CreatedAt            time.Time      `gorm:"autoCreateTime" json:"created_at"`
+	UpdatedAt            time.Time      `gorm:"autoUpdateTime" json:"updated_at"`
+
+	Inbox model.Inbox `gorm:"foreignKey:InboxID" json:"inbox,omitempty"`
+}
+
+func (ChannelAPI) TableName() string                        { return "channel_api" }
+func (c ChannelAPI) GetInboxID() uint                       { return c.InboxID }
+func (c ChannelAPI) GetChannelType() model.InboxChannelType { return model.InboxChannelTypeAPI }