feat(deploy): fix docker deployment and add backend i18n

- Docker Deployment Fixes:
  - Switch base images to docker.m.daocloud.io to resolve registry 401 errors
  - Add Postgres and Redis services to docker-compose.traefik.yml
  - Fix frontend build: replace missing icons (Globe->Location, Chart->TrendCharts)
  - Fix frontend build: resolve pnpm CI/TTY issues and frozen lockfile errors
  - Add missing backend dependencies (sqlalchemy, psycopg2, redis-py, celery, docker-py) in pixi.toml
  - Ensure database tables are created on startup (lifespan event)

- Backend Internationalization (i18n):
  - Add backend/app/core/i18n.py for locale handling
  - Update API endpoints (jobs, tasks, uploads, results) to return localized messages
  - Support 'Accept-Language' header (en/zh)

- Documentation:
  - Update DOCKER_DEPLOYMENT.md with new architecture and troubleshooting
  - Update AGENTS.md with latest stack details and deployment steps
  - Update @fix_plan.md status

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/utils/i18n.py

@@ -0,0 +1,71 @@
+from typing import Dict, Optional
+from fastapi import Header
+
+# Translation dictionaries
+TRANSLATIONS = {
+    "en": {
+        "job_not_found": "Job not found",
+        "invalid_extension": "Invalid file extension: {ext}. Allowed: {allowed}",
+        "single_file_only": "Only one file allowed per task",
+        "queue_update_triggered": "Queue update triggered",
+        "task_created": "Task created successfully",
+        "server_error": "Internal server error",
+        "job_not_completed": "Job not completed. Current status: {status}",
+        "results_not_found": "Results not found on disk",
+        "job_deleted": "Job {job_id} deleted successfully",
+        "use_post_jobs": "Use POST /api/v1/jobs/create for now",
+        "task_not_found": "Task not found",
+        "task_not_in_queue": "Task is not in queue",
+    },
+    "zh": {
+        "job_not_found": "任务不存在",
+        "invalid_extension": "文件后缀无效: {ext}。允许: {allowed}",
+        "single_file_only": "每次任务只能上传一个文件",
+        "queue_update_triggered": "已触发队列更新",
+        "task_created": "任务创建成功",
+        "server_error": "服务器内部错误",
+        "job_not_completed": "任务未完成。当前状态: {status}",
+        "results_not_found": "未找到结果文件",
+        "job_deleted": "任务 {job_id} 已删除",
+        "use_post_jobs": "请使用 POST /api/v1/jobs/create 接口",
+        "task_not_found": "任务不存在",
+        "task_not_in_queue": "任务不在队列中",
+    }
+}
+
+DEFAULT_LANG = "en"
+
+def get_language(accept_language: Optional[str] = Header(None)) -> str:
+    """
+    Determine the preferred language from the Accept-Language header.
+    Defaults to 'en' if not specified or not supported.
+    """
+    if not accept_language:
+        return DEFAULT_LANG
+
+    # Simple parsing: check if 'zh' is in the header
+    # A more robust parser could be added if needed
+    if "zh" in accept_language.lower():
+        return "zh"
+
+    return DEFAULT_LANG
+
+def get_text(key: str, lang: str = DEFAULT_LANG, **kwargs) -> str:
+    """
+    Get translated text for a given key and language.
+    Supports string formatting with kwargs.
+    """
+    # Fallback to default language if lang not supported
+    if lang not in TRANSLATIONS:
+        lang = DEFAULT_LANG
+
+    # Fallback to key if message not found in language
+    message = TRANSLATIONS[lang].get(key)
+    if not message:
+        # Try default language
+        message = TRANSLATIONS[DEFAULT_LANG].get(key, key)
+
+    try:
+        return message.format(**kwargs)
+    except KeyError:
+        return message