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/article_embedding.go

@@ -0,0 +1,18 @@
+package model
+
+import (
+	"encoding/json"
+)
+
+// ArticleEmbedding stores vector embeddings for semantic article search.
+// Reference: Chatwoot ArticleEmbedding (enterprise) + P2B M9 spec
+type ArticleEmbedding struct {
+	Base
+	ArticleID uint            `gorm:"not null;index" json:"article_id"`
+	Embedding json.RawMessage `gorm:"type:jsonb" json:"embedding"` // vector data (1536-dim, stored as JSON for portability)
+	Term      string          `gorm:"type:text;not null" json:"term"` // searchable text content
+
+	Article Article `gorm:"foreignKey:ArticleID" json:"article,omitempty"`
+}
+
+func (ArticleEmbedding) TableName() string { return "article_embeddings" }