chore(async-infra): 异步基础层 + 工具链 + 清理

异步基础层（core/）：
- db.py   aiosqlite + WAL：读路径无锁并发、写路径 asyncio.Lock 串行，
          busy_timeout 自旋——实现「读写同时进行」（旧实现用 RLock 全读串行）
- http_client.py  aiohttp 连接池 + 有界重试（5xx 重试 / 4xx 不重试）+
          响应体上限 + 流式接口；按事件循环隔离 session（支持迁移期双 loop）

工具链：
- pyproject.toml   ruff(E/F/W/I/B/UP/C4/ASYNC/S/RUF) + pyright(basic) + pytest(asyncio auto)
- requirements-fastapi.txt  FastAPI/aiohttp/aiosqlite/aiofiles 等目标栈
- requirements-dev.txt      开发依赖（pytest/ruff/pyright）

清理：删除无用脚本 _csscheck.py 与产物标记 _css_ok.txt/_pkg_ok.txt，同步 .gitignore

验证：pytest 10/10（含「写 200 条同时 4 读协程零异常」并发用例）；
ruff All checks passed；pyright 0 errors。

tests/test_db.py

@@ -0,0 +1,79 @@
+# -*- coding: utf-8 -*-
+"""异步 SQLite 层测试：读写并发、WAL、事务原子性。"""
+
+import asyncio
+
+import pytest
+
+import core.db as db
+
+
+@pytest.fixture()
+def temp_db(tmp_path, monkeypatch):
+    """把 DB 指向临时目录，独立于生产 data/site.db。"""
+    monkeypatch.setattr(db, 'DATA_DIR', tmp_path)
+    monkeypatch.setattr(db, 'DB_PATH', tmp_path / 'site.db')
+    return tmp_path / 'site.db'
+
+
+async def test_write_then_read(temp_db):
+    await db.execute('CREATE TABLE t (id INTEGER PRIMARY KEY, v TEXT)')
+    rid = await db.execute('INSERT INTO t (v) VALUES (?)', ('hello',))
+    assert rid == 1
+    row = await db.fetch_one('SELECT v FROM t WHERE id=?', (1,))
+    assert row['v'] == 'hello'
+    assert await db.fetch_val('SELECT COUNT(*) FROM t') == 1
+
+
+async def test_wal_mode_enabled(temp_db):
+    await db.execute('CREATE TABLE t (id INTEGER PRIMARY KEY)')
+    mode = await db.fetch_val('PRAGMA journal_mode')
+    assert str(mode).lower() == 'wal'
+
+
+async def test_concurrent_read_write(temp_db):
+    """核心诉求：写的同时读不被阻塞、且读到一致快照（WAL 快照隔离）。"""
+    await db.execute('CREATE TABLE nums (id INTEGER PRIMARY KEY, v INTEGER)')
+
+    stop = asyncio.Event()
+    read_errors = []
+    write_count = 0
+
+    async def writer():
+        nonlocal write_count
+        for i in range(200):
+            await db.execute('INSERT INTO nums (v) VALUES (?)', (i,))
+            write_count += 1
+        stop.set()
+
+    async def reader():
+        # 写入过程中持续读，不应抛 SQLITE_BUSY / 不应被阻塞致死
+        while not stop.is_set():
+            try:
+                n = await db.fetch_val('SELECT COUNT(*) FROM nums')
+                assert isinstance(n, int)
+            except Exception as e:
+                read_errors.append(e)
+            await asyncio.sleep(0)
+
+    await asyncio.gather(writer(), *[reader() for _ in range(4)])
+
+    assert not read_errors, f'读期间出现异常: {read_errors[:3]}'
+    assert write_count == 200
+    assert await db.fetch_val('SELECT COUNT(*) FROM nums') == 200
+
+
+async def test_write_transaction_commit_and_rollback(temp_db):
+    await db.execute('CREATE TABLE t (id INTEGER PRIMARY KEY, v TEXT)')
+
+    async with db.write_transaction() as conn:
+        await conn.execute('INSERT INTO t (v) VALUES (?)', ('a',))
+        await conn.execute('INSERT INTO t (v) VALUES (?)', ('b',))
+    assert await db.fetch_val('SELECT COUNT(*) FROM t') == 2
+
+    with pytest.raises(RuntimeError):
+        async with db.write_transaction() as conn:
+            await conn.execute('INSERT INTO t (v) VALUES (?)', ('c',))
+            raise RuntimeError('boom')
+    # 回滚：c 不应落库
+    assert await db.fetch_val('SELECT COUNT(*) FROM t') == 2