fix: alembic version_num 长度修复 + 前端部署配置

- env.py: 自动创建 alembic_version 表时使用 VARCHAR(255) 避免截断
- docker-compose: 添加 frontend nginx 服务 + alembic 挂载
- nginx.conf: SPA 路由 + API 代理

alembic/env.py

@@ -3,7 +3,7 @@ from logging.config import fileConfig
 import sys
 from pathlib import Path
 
-from sqlalchemy import engine_from_config, pool
+from sqlalchemy import engine_from_config, pool, text
 from alembic import context
 
 # 把项目根加入 pythonpath,让 alembic 能找到 src 包
@@ -29,6 +29,23 @@ if config.config_file_name is not None:
 target_metadata = Base.metadata
 
 
+def _ensure_version_table(connection):
+    """确保 alembic_version 表存在且 version_num 列足够长。
+
+    Alembic 默认 version_num 是 String(32),但我们的迁移名较长(如
+    0005_prediction_status_and_stats_provenance 有 41 个字符),会导致
+    StringDataRightTruncation 错误。
+    """
+    result = connection.execute(text(
+        "SELECT EXISTS (SELECT 1 FROM information_schema.tables WHERE table_name = 'alembic_version')"
+    ))
+    if not result.scalar():
+        connection.execute(text(
+            "CREATE TABLE alembic_version (version_num VARCHAR(255) NOT NULL PRIMARY KEY)"
+        ))
+        connection.commit()
+
+
 def run_migrations_offline() -> None:
     """Run migrations in 'offline' mode."""
     url = config.get_main_option("sqlalchemy.url")
@@ -52,6 +69,7 @@ def run_migrations_online() -> None:
     )
 
     with connectable.connect() as connection:
+        _ensure_version_table(connection)
         context.configure(connection=connection, target_metadata=target_metadata)
 
         with context.begin_transaction():