feat(db): 引入 Alembic 取代裸 DDL 迁移

- alembic init + env.py 接 settings/models metadata（asyncpg->psycopg2 同步 URL）
- 补 3 个 CheckConstraint 到 models（原只在裸 DDL）
- 离线生成初始迁移（31 表+约束+95 索引，全 sa.* 通用类型）
- init_db 用 _run_alembic_migrations（自动基线+upgrade head）替换 create_tables+ensure_schema_updates（删 92 行裸 DDL）
- 删破坏性 migrate_db.py（drop_all）
- 既有 DB 首次启动自动 stamp 基线，无需手动

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

alembic/script.py.mako

@@ -0,0 +1,28 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}