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/core/i18n.py

@@ -0,0 +1,68 @@
+from fastapi import Request
+from typing import Dict, Any, Optional
+
+# Translation dictionaries
+TRANSLATIONS = {
+    "en": {
+        "job_not_found": "Job not found",
+        "task_not_found": "Task not found",
+        "invalid_extension": "Invalid file extension: {ext}. Allowed: {allowed}",
+        "single_file_only": "Only one file allowed per task",
+        "job_not_completed": "Job not completed. Current status: {status}",
+        "results_not_found": "Results not found on disk",
+        "job_deleted": "Job {job_id} deleted successfully",
+        "task_not_in_queue": "Task is not in queue",
+        "use_create_endpoint": "Use POST /api/v1/jobs/create for now",
+        "upload_endpoint": "Upload endpoint",
+        "queue_update_triggered": "Queue update triggered"
+    },
+    "zh": {
+        "job_not_found": "未找到任务",
+        "task_not_found": "未找到任务",
+        "invalid_extension": "文件后缀无效: {ext}。允许: {allowed}",
+        "single_file_only": "每个任务仅允许上传一个文件",
+        "job_not_completed": "任务未完成。当前状态: {status}",
+        "results_not_found": "未在磁盘上找到结果文件",
+        "job_deleted": "任务 {job_id} 已成功删除",
+        "task_not_in_queue": "任务不在队列中",
+        "use_create_endpoint": "请使用 POST /api/v1/jobs/create 接口",
+        "upload_endpoint": "上传接口",
+        "queue_update_triggered": "已触发队列更新"
+    }
+}
+
+DEFAULT_LOCALE = "en"
+
+def get_locale(request: Request) -> str:
+    """
+    Get locale from Accept-Language header.
+    Simple implementation: checks if 'zh' is in the header.
+    """
+    accept_language = request.headers.get("accept-language", "")
+    if "zh" in accept_language.lower():
+        return "zh"
+    return "en"
+
+class I18n:
+    def __init__(self, request: Request):
+        self.locale = get_locale(request)
+
+    def t(self, key: str, **kwargs) -> str:
+        """
+        Translate a key.
+        If key not found, return key.
+        Supports string formatting with kwargs.
+        """
+        messages = TRANSLATIONS.get(self.locale, TRANSLATIONS[DEFAULT_LOCALE])
+        message = messages.get(key, TRANSLATIONS[DEFAULT_LOCALE].get(key, key))
+
+        if kwargs:
+            try:
+                return message.format(**kwargs)
+            except KeyError:
+                return message
+        return message
+
+async def get_i18n(request: Request) -> I18n:
+    """Dependency for getting I18n instance"""
+    return I18n(request)