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/service/note_service.go

@@ -0,0 +1,84 @@
+package service
+
+// NoteService provides business logic for contact notes.
+// Reference: Chatwoot app/controllers/api/v1/accounts/contacts/notes_controller.rb
+
+import (
+	"errors"
+
+	"github.com/gochat/gochat/internal/model"
+	"github.com/gochat/gochat/internal/repository"
+)
+
+var (
+	ErrNoteNotFound      = errors.New("note not found")
+	ErrNoteContentEmpty  = errors.New("note content is required")
+	ErrNoteAccountMismatch = errors.New("note does not belong to the account")
+)
+
+// NoteService handles CRUD operations for contact notes.
+type NoteService struct {
+	noteRepo *repository.NoteRepo
+}
+
+// NewNoteService creates a new NoteService instance.
+func NewNoteService(noteRepo *repository.NoteRepo) *NoteService {
+	return &NoteService{noteRepo: noteRepo}
+}
+
+// List returns all notes for a contact in an account, ordered latest first.
+// Reference: Chatwoot `def index; @notes = @contact.notes.latest.includes(:user); end`
+func (s *NoteService) List(accountID, contactID uint) ([]model.Note, error) {
+	return s.noteRepo.ListByContact(accountID, contactID)
+}
+
+// Get returns a single note by ID.
+// Reference: Chatwoot `def show; end`
+func (s *NoteService) Get(accountID, contactID, noteID uint) (*model.Note, error) {
+	note, err := s.noteRepo.GetByID(accountID, contactID, noteID)
+	if err != nil {
+		return nil, ErrNoteNotFound
+	}
+	return note, nil
+}
+
+// Create creates a new note for a contact.
+// Reference: Chatwoot `def create; @note = @contact.notes.create!(note_params); end`
+// note_params merges contact_id and current_user.id
+func (s *NoteService) Create(accountID, contactID uint, userID uint, content string) (*model.Note, error) {
+	if content == "" {
+		return nil, ErrNoteContentEmpty
+	}
+	note := &model.Note{
+		Content:   content,
+		AccountID: accountID,
+		ContactID: contactID,
+		UserID:    &userID,
+	}
+	return s.noteRepo.CreateNote(note)
+}
+
+// Update updates an existing note's content.
+// Reference: Chatwoot `def update; @note.update(note_params); end`
+func (s *NoteService) Update(accountID, contactID, noteID uint, content string) (*model.Note, error) {
+	if content == "" {
+		return nil, ErrNoteContentEmpty
+	}
+	note, err := s.noteRepo.GetByID(accountID, contactID, noteID)
+	if err != nil {
+		return nil, ErrNoteNotFound
+	}
+	note.Content = content
+	return s.noteRepo.Update(note)
+}
+
+// Delete removes a note.
+// Reference: Chatwoot `def destroy; @note.destroy!; head :ok; end`
+// Chatwoot returns 200 OK on destroy, not 204 NoContent.
+func (s *NoteService) Delete(accountID, contactID, noteID uint) error {
+	err := s.noteRepo.Delete(accountID, contactID, noteID)
+	if err != nil {
+		return ErrNoteNotFound
+	}
+	return nil
+}