feat(db-async): 站点数据异步读写仓储 + FastAPI 生命周期

- core/repo.py：经由 core.db(aiosqlite) 读写 auth_system 的**同一份** site.db
  （settings/announcements/users 只读/ledger 只读），字段口径对齐 auth_system；
  读并发、写串行、WAL 读写同时进行
- web/routers/site.py：/api/v1/site/announcements、/api/v1/site/stats（异步多表并发读）
- web/app.py：lifespan 关闭时释放 aiohttp 连接池（含按 loop 隔离的 session）

写路径说明：迁移期写操作仍归 auth_system 的 _DB_LOCK 事务口径，避免两套写
路径互相踩踏；本层先只做读。

验证：pytest 22/22（新增 6 项含「异步读 + 同步写者并发 WAL」）；
/api/v1/site/* 实测读到共享库（3 用户/1 管理员）；ruff/pyright 干净。

tests/test_repo.py

@@ -0,0 +1,96 @@
+# -*- coding: utf-8 -*-
+"""异步仓储层测试：读写共享库、并发读、与 auth_system schema 兼容。"""
+import asyncio
+import sqlite3
+
+import pytest
+
+import core.db as db
+import core.repo as repo
+
+
+@pytest.fixture()
+def site_db(tmp_path, monkeypatch):
+    """建一个与 auth_system 口径一致的迷你库，指给 core.db。"""
+    path = tmp_path / 'site.db'
+    conn = sqlite3.connect(str(path))
+    conn.executescript("""
+        CREATE TABLE users (id INTEGER PRIMARY KEY, username TEXT UNIQUE, role TEXT DEFAULT 'user');
+        CREATE TABLE settings (key TEXT PRIMARY KEY, value TEXT DEFAULT '');
+        CREATE TABLE announcements (id INTEGER PRIMARY KEY, title TEXT DEFAULT '', content TEXT DEFAULT '',
+                                    popup_mode TEXT DEFAULT 'off', updated_at TEXT DEFAULT '');
+        CREATE TABLE billing_ledger (task_id TEXT PRIMARY KEY, user_id INTEGER, status TEXT DEFAULT 'open');
+        INSERT INTO users (username, role) VALUES ('alice','user'), ('bob','user'), ('qfbsh','admin');
+        INSERT INTO settings (key, value) VALUES ('server_nickname','云村测试');
+        INSERT INTO announcements (id, title, content) VALUES (1, '公告', '正文');
+        INSERT INTO billing_ledger (task_id, user_id, status) VALUES ('t1', 1, 'open');
+    """)
+    conn.commit()
+    conn.close()
+    monkeypatch.setattr(db, 'DATA_DIR', tmp_path)
+    monkeypatch.setattr(db, 'DB_PATH', path)
+    return path
+
+
+async def test_get_setting(site_db):
+    assert await repo.get_setting('server_nickname') == '云村测试'
+    assert await repo.get_setting('missing', 'fallback') == 'fallback'
+
+
+async def test_set_setting_upsert(site_db):
+    await repo.set_setting('a', '1')
+    assert await repo.get_setting('a') == '1'
+    await repo.set_setting('a', '2')
+    assert await repo.get_setting('a') == '2'
+
+
+async def test_counts(site_db):
+    assert await repo.count_users() == 3
+    assert await repo.count_users('admin') == 1
+    assert await repo.ledger_open_count() == 1
+
+
+async def test_list_announcements(site_db):
+    rows = await repo.list_announcements()
+    assert rows and rows[0]['title'] == '公告'
+
+
+async def test_concurrent_reads(site_db):
+    """并发读同一库不互相阻塞、结果一致。"""
+    results = await asyncio.gather(*[repo.count_users() for _ in range(20)])
+    assert results == [3] * 20
+
+
+async def test_read_write_concurrent_with_sync_writer(site_db):
+    """异步读与『另一个同步写者』（模拟遗留栈写库）可同时进行（WAL）。"""
+    stop = asyncio.Event()
+    errors = []
+
+    def sync_writer():
+        # 模拟 auth_system 的同步写：独立连接持续写 settings
+        conn = sqlite3.connect(str(site_db), timeout=15)
+        conn.execute('PRAGMA journal_mode=WAL')
+        try:
+            for i in range(80):
+                conn.execute("INSERT INTO settings (key, value) VALUES (?, 'x') "
+                             "ON CONFLICT(key) DO UPDATE SET value='x'", (f'k{i}',))
+                conn.commit()
+        finally:
+            conn.close()
+            stop.set()
+
+    async def reader():
+        while not stop.is_set():
+            try:
+                await repo.get_setting('server_nickname')
+            except Exception as e:
+                errors.append(e)
+            await asyncio.sleep(0)
+
+    # sync_writer 放线程池，reader 留在事件循环
+    loop = asyncio.get_running_loop()
+    writer_task = loop.run_in_executor(None, sync_writer)
+    await asyncio.gather(writer_task, *[reader() for _ in range(3)])
+
+    assert not errors, f'读期间异常: {errors[:3]}'
+    assert await repo.get_setting('k79') == 'x'