feat: 新人初始礼包购买追踪功能

- 数据库：添加 has_purchased_starter_pack 字段到 register_bonus_claims
- 后端：创建静态配置管理套餐信息，支持按国家/地区区分
- 后端：新增 GET /api/v1/points/packages API 返回可用套餐
- 后端：创建 utils/paths.py 统一路径管理
- 前端：动态获取套餐信息，移除硬编码
- 前端：添加 ProductCode 枚举约束，前后端类型安全
- 配置：Profile 默认国家改为 US（ISO 3166-1 alpha-2）
- 文档：更新协议文档说明新 API 和字段

backend/alembic/versions/20260416_0001_add_starter_pack_tracking.py

@@ -0,0 +1,32 @@
+"""add has_purchased_starter_pack to register_bonus_claims
+
+Revision ID: 20260416_0001
+Revises: 20260413_0004
+Create Date: 2026-04-16 12:00:00
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "20260416_0001"
+down_revision: Union[str, Sequence[str], None] = "20260415_0002"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "register_bonus_claims",
+        sa.Column(
+            "has_purchased_starter_pack",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.text("false"),
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("register_bonus_claims", "has_purchased_starter_pack")