fix: исправить RuntimeError в aiosqlite — _get_conn как async context manager

`async with await _get_conn()` запускал тред дважды: первый раз внутри
`_get_conn` через `await aiosqlite.connect()`, второй раз в `__aenter__`
через `await self`. Преобразован в `@asynccontextmanager` с `yield` и
`finally: conn.close()`. Все вызывающие места обновлены. Тест
`test_init_db_synchronous` обновлён под новый API.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/db.py

@@ -1,22 +1,27 @@
 from __future__ import annotations
 
-from typing import Optional
+from contextlib import asynccontextmanager
+from typing import AsyncGenerator, Optional
 import aiosqlite
 
 from backend import config
 
 
-async def _get_conn() -> aiosqlite.Connection:
+@asynccontextmanager
+async def _get_conn() -> AsyncGenerator[aiosqlite.Connection, None]:
     conn = await aiosqlite.connect(config.DB_PATH)
     await conn.execute("PRAGMA journal_mode=WAL")
     await conn.execute("PRAGMA busy_timeout=5000")
     await conn.execute("PRAGMA synchronous=NORMAL")
     conn.row_factory = aiosqlite.Row
-    return conn
+    try:
+        yield conn
+    finally:
+        await conn.close()
 
 
 async def init_db() -> None:
-    async with await _get_conn() as conn:
+    async with _get_conn() as conn:
         await conn.executescript("""
             CREATE TABLE IF NOT EXISTS users (
                 id         INTEGER PRIMARY KEY AUTOINCREMENT,
@@ -57,7 +62,7 @@ async def init_db() -> None:
 
 
 async def register_user(uuid: str, name: str) -> dict:
-    async with await _get_conn() as conn:
+    async with _get_conn() as conn:
         await conn.execute(
             "INSERT OR IGNORE INTO users (uuid, name) VALUES (?, ?)",
             (uuid, name),
@@ -77,7 +82,7 @@ async def save_signal(
     lon: Optional[float],
     accuracy: Optional[float],
 ) -> int:
-    async with await _get_conn() as conn:
+    async with _get_conn() as conn:
         async with conn.execute(
             """
             INSERT INTO signals (user_uuid, timestamp, lat, lon, accuracy)
@@ -91,7 +96,7 @@ async def save_signal(
 
 
 async def get_user_name(uuid: str) -> Optional[str]:
-    async with await _get_conn() as conn:
+    async with _get_conn() as conn:
         async with conn.execute(
             "SELECT name FROM users WHERE uuid = ?", (uuid,)
         ) as cur:
@@ -104,7 +109,7 @@ async def save_telegram_batch(
     signals_count: int,
     signal_ids: list[int],
 ) -> int:
-    async with await _get_conn() as conn:
+    async with _get_conn() as conn:
         async with conn.execute(
             """
             INSERT INTO telegram_batches (message_text, sent_at, signals_count, status)