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/category.go

@@ -0,0 +1,31 @@
+package model
+
+import (
+	"encoding/json"
+)
+
+// Category represents a help center category.
+// Reference: Chatwoot Category model + P2B M9 spec
+type Category struct {
+	Base
+	AccountID           uint            `gorm:"index;not null" json:"account_id"`
+	PortalID            uint            `gorm:"index;not null" json:"portal_id"`
+	Name                string          `gorm:"size:255;not null" json:"name"`
+	Slug                string          `gorm:"size:255;not null;index" json:"slug"`
+	Description         string          `gorm:"type:text" json:"description"`
+	Icon                string          `gorm:"size:255;default:''" json:"icon"`
+	Position            int             `gorm:"default:0" json:"position"`
+	Locale              string          `gorm:"size:10;default:'en';not null" json:"locale"`
+	ParentID            *uint           `gorm:"index" json:"parent_id,omitempty"`
+	AssociatedCategoryID *uint          `gorm:"index" json:"associated_category_id,omitempty"` // cross-locale link
+	CustomAttributes    json.RawMessage `gorm:"type:jsonb;serializer:json" json:"custom_attributes"`
+
+	Portal            Portal           `gorm:"foreignKey:PortalID" json:"portal,omitempty"`
+	Parent            *Category        `gorm:"foreignKey:ParentID" json:"parent,omitempty"`
+	AssociatedCategory *Category       `gorm:"foreignKey:AssociatedCategoryID" json:"associated_category,omitempty"`
+	Articles          []Article        `gorm:"foreignKey:CategoryID" json:"articles,omitempty"`
+	Folders           []Folder         `gorm:"foreignKey:CategoryID" json:"folders,omitempty"`
+	RelatedCategories []RelatedCategory `gorm:"foreignKey:CategoryID" json:"related_categories,omitempty"`
+}
+
+func (Category) TableName() string { return "categories" }