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/conversation_label.go

@@ -0,0 +1,23 @@
+package model
+
+import (
+	"time"
+)
+
+// ConversationLabel represents a label/tag applied to a conversation.
+// Reference: Chatwoot tag join model + P2B M3 spec
+// This is the join table between Conversation and Tag, replacing the
+// old comma-separated Labels string field on Conversation.
+type ConversationLabel struct {
+	ID             uint      `gorm:"primaryKey;autoIncrement" json:"id"`
+	ConversationID uint      `gorm:"not null;uniqueIndex:idx_conv_label_tag;index" json:"conversation_id"`
+	TagID          uint      `gorm:"not null;uniqueIndex:idx_conv_label_tag;index" json:"tag_id"`
+	AccountID      uint      `gorm:"not null;index" json:"account_id"`
+	CreatedAt      time.Time `gorm:"autoCreateTime" json:"created_at"`
+
+	Conversation Conversation `gorm:"foreignKey:ConversationID" json:"conversation,omitempty"`
+	Tag          Tag          `gorm:"foreignKey:TagID" json:"tag,omitempty"`
+	Account      Account      `gorm:"foreignKey:AccountID" json:"account,omitempty"`
+}
+
+func (ConversationLabel) TableName() string { return "conversation_labels" }