perf(*): bound list queries, batch fetches, harden media downloads

backend/src/utils/read/accounts.py

@@ -30,10 +30,19 @@ RETURNING {_ACCOUNT_COLS}
 """  # noqa: S608
 
 
+_self_ids: dict[int, int] = {}
+
+
 async def self_user_id(pool: asyncpg.Pool, account_id: int) -> int | None:
-    return await pool.fetchval(
+    cached = _self_ids.get(account_id)
+    if cached is not None:
+        return cached
+    tg_user_id = await pool.fetchval(
         "SELECT tg_user_id FROM accounts WHERE account_id = $1", account_id
     )
+    if tg_user_id is not None:
+        _self_ids[account_id] = tg_user_id
+    return tg_user_id
 
 
 async def list_accounts(pool: asyncpg.Pool) -> list[AccountView]: