Implement Alembic for database migrations and update FastAPI initialization

- Integrate Alembic for managing database schema migrations, replacing the previous SQL schema management.
- Update the FastAPI application to synchronize interface definitions at startup.
- Modify the Docker Compose command to run Alembic migrations before starting the API.
- Enhance the Makefile with new commands for database migration and revision management.
- Remove outdated SQL schema and seed files, transitioning to a more dynamic migration approach.
- Add initial migration scripts and configuration for Alembic, ensuring a structured database evolution.

backend/app.py

@@ -1,4 +1,4 @@
-"""FastAPI 入口。挂载路由,放行前端跨域,启动时建表。
+"""FastAPI 入口。挂载路由,放行前端跨域,启动时同步接口定义。
 
 启动:  uv run --with-requirements requirements.txt uvicorn app:app --reload --port 8000
 
@@ -16,7 +16,7 @@ from contextlib import asynccontextmanager
 
 import config
 import uvicorn
-from db.session import init_db
+from db.session import sync_interface_definitions
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
@@ -33,7 +33,7 @@ from routes import (
 
 @asynccontextmanager
 async def lifespan(_app: FastAPI):
-    await init_db()  # MVP:启动建表;表稳定后切 alembic
+    await sync_interface_definitions()
     yield