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/channelable.go

@@ -0,0 +1,27 @@
+package model
+
+// Channelable defines the interface for polymorphic channel configurations.
+// Each inbox has a channel_type (e.g. "Channel::WebWidget") and an associated
+// channel model that implements this interface.
+// Reference: P2B M2 polymorphic channel pattern
+type Channelable interface {
+	// GetInboxID returns the InboxID foreign key for this channel.
+	GetInboxID() uint
+	// GetChannelType returns the channel type string (e.g. "Channel::WebWidget").
+	GetChannelType() InboxChannelType
+	// TableName returns the GORM table name for this channel model.
+	TableName() string
+}
+
+// ChannelRegistry maps channel type strings to factory functions that create Channelable instances.
+// This map is populated during app initialization (in app.go) to avoid import cycles
+// between the model and model/channel packages.
+// Usage: register from app.go via model.RegisterChannelFactory(...)
+var ChannelRegistry = map[string]func() Channelable{}
+
+// RegisterChannelFactory adds a channel type factory to the registry.
+// Called from app.go during initialization to wire up model/channel types
+// without creating a circular import between model ↔ model/channel.
+func RegisterChannelFactory(channelType string, factory func() Channelable) {
+	ChannelRegistry[channelType] = factory
+}