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.go

@@ -0,0 +1,36 @@
+package model
+
+import (
+	"encoding/json"
+)
+
+// Article represents a help center article.
+// Reference: Chatwoot Article model + P2B M9 spec
+type Article struct {
+	Base
+	AccountID           uint            `gorm:"index;not null" json:"account_id"`
+	PortalID            uint            `gorm:"index;not null" json:"portal_id"`
+	CategoryID          *uint           `gorm:"index" json:"category_id,omitempty"`
+	FolderID            *uint           `gorm:"index" json:"folder_id,omitempty"`
+	AuthorID            *uint           `gorm:"index" json:"author_id,omitempty"`
+	Title               string          `gorm:"size:255;not null" json:"title"`
+	Slug                string          `gorm:"size:255;not null;uniqueIndex" json:"slug"`
+	Description         string          `gorm:"type:text" json:"description"` // short description for list display + search weight B
+	Content             string          `gorm:"type:text" json:"content"`
+	Status              string          `gorm:"size:50;not null;default:'draft'" json:"status"` // draft/published/archived (ArticleStatus)
+	Position            int             `gorm:"default:0" json:"position"`
+	Views               int             `gorm:"default:0" json:"views"`
+	Locale              string          `gorm:"size:10;default:'en';not null" json:"locale"`
+	AssociatedArticleID *uint           `gorm:"index" json:"associated_article_id,omitempty"` // translations: root article
+	Meta                json.RawMessage `gorm:"type:jsonb;serializer:json" json:"meta"`
+	CustomAttributes    json.RawMessage `gorm:"type:jsonb;serializer:json" json:"custom_attributes"`
+
+	Portal             Portal    `gorm:"foreignKey:PortalID" json:"portal,omitempty"`
+	Category           *Category `gorm:"foreignKey:CategoryID" json:"category,omitempty"`
+	Folder             *Folder   `gorm:"foreignKey:FolderID" json:"folder,omitempty"`
+	Author             *User     `gorm:"foreignKey:AuthorID" json:"author,omitempty"`
+	AssociatedArticle  *Article  `gorm:"foreignKey:AssociatedArticleID" json:"associated_article,omitempty"`
+	AssociatedArticles []Article `gorm:"foreignKey:AssociatedArticleID" json:"associated_articles,omitempty"`
+}
+
+func (Article) TableName() string { return "articles" }