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/scripts/migrate.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+# GoChat Database Migration Script
+# Reference: Chatwoot Rails db:migrate pattern
+# Uses golang-migrate for SQL migrations
+
+set -e
+
+MIGRATIONS_DIR="./migrations"
+DB_HOST="${POSTGRES_HOST:-localhost}"
+DB_PORT="${POSTGRES_PORT:-5432}"
+DB_NAME="${POSTGRES_DATABASE:-gochat_dev}"
+DB_USER="${POSTGRES_USERNAME:-postgres}"
+DB_PASS="${POSTGRES_PASSWORD:-}"
+
+DSN="postgres://${DB_USER}:${DB_PASS}@${DB_HOST}:${DB_PORT}/${DB_NAME}?sslmode=disable"
+
+case "$1" in
+  create)
+    echo "Creating database ${DB_NAME}..."
+    createdb -h "${DB_HOST}" -p "${DB_PORT}" -U "${DB_USER}" "${DB_NAME}" 2>/dev/null || echo "Database already exists"
+    ;;
+  up)
+    echo "Running migrations up..."
+    migrate -path "${MIGRATIONS_DIR}" -database "${DSN}" up
+    ;;
+  down)
+    echo "Running migrations down..."
+    migrate -path "${MIGRATIONS_DIR}" -database "${DSN}" down
+    ;;
+  force)
+    echo "Force migration version..."
+    migrate -path "${MIGRATIONS_DIR}" -database "${DSN}" force "$2"
+    ;;
+  version)
+    echo "Current migration version:"
+    migrate -path "${MIGRATIONS_DIR}" -database "${DSN}" version
+    ;;
+  *)
+    echo "Usage: $0 {create|up|down|force|version}"
+    exit 1
+    ;;
+esac