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.

deploy/docker/docker-compose.prod.yml

@@ -0,0 +1,107 @@
+# GoChat Production Environment
+# Reference: Chatwoot docker-compose.production.yaml — rails + sidekiq + postgres + redis, restart policies
+# Enhanced with pgvector, resource limits, non-root user, health checks
+
+version: '3.8'
+
+services:
+  postgres:
+    image: pgvector/pgvector:pg16
+    container_name: gochat-postgres
+    restart: always
+    environment:
+      POSTGRES_DB: ${POSTGRES_DB:-gochat_production}
+      POSTGRES_USER: ${POSTGRES_USER:-gochat}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}  # MUST be set in .env
+    ports:
+      - "127.0.0.1:5432:5432"  # Only localhost access
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-gochat}"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+    deploy:
+      resources:
+        limits:
+          memory: 1G
+
+  redis:
+    image: redis:7-alpine
+    container_name: gochat-redis
+    restart: always
+    command: redis-server --requirepass "${REDIS_PASSWORD}" --appendonly yes --maxmemory 512mb --maxmemory-policy allkeys-lru
+    ports:
+      - "127.0.0.1:6379:6379"  # Only localhost access
+    volumes:
+      - redis_data:/data
+    healthcheck:
+      test: ["CMD", "redis-cli", "-a", "${REDIS_PASSWORD}", "ping"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+    deploy:
+      resources:
+        limits:
+          memory: 512M
+
+  gochat:
+    image: gochat/gochat:${GOCHAT_VERSION:-latest}
+    container_name: gochat-app
+    restart: always
+    depends_on:
+      postgres:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+    ports:
+      - "127.0.0.1:3000:3000"  # Reverse proxy should handle external access
+    env_file: ../../.env
+    environment:
+      - GOCHAT_ENV=production
+      - POSTGRES_HOST=postgres
+      - POSTGRES_PORT=5432
+      - REDIS_HOST=redis
+      - REDIS_PORT=6379
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:3000/health"]
+      interval: 30s
+      timeout: 5s
+      start_period: 15s
+      retries: 3
+    deploy:
+      resources:
+        limits:
+          memory: 512M
+          cpus: '1.0'
+        reservations:
+          memory: 256M
+          cpus: '0.5'
+
+  worker:
+    image: gochat/gochat:${GOCHAT_VERSION:-latest}
+    container_name: gochat-worker
+    restart: always
+    depends_on:
+      postgres:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+    command: ["serve", "--worker-only"]
+    env_file: ../../.env
+    environment:
+      - GOCHAT_ENV=production
+      - POSTGRES_HOST=postgres
+      - POSTGRES_PORT=5432
+      - REDIS_HOST=redis
+      - REDIS_PORT=6379
+    deploy:
+      resources:
+        limits:
+          memory: 512M
+          cpus: '1.0'
+
+volumes:
+  postgres_data:
+  redis_data: