fix: 18-point audit pass — payment races, hwid limit bugs, blocking SSH/HTTP in event loops, N+1 queries, ssh host-key pinning, dead code

payments: _grant_paid_subscription now validates plan/node exist before
marking a payment paid instead of after (was leaving charged-but-ungranted
payments with no error trail); mark_payment_paid is now a single atomic
UPDATE ... WHERE status='pending' instead of check-then-act, closing a
double-grant race between webhooks and the periodic reconciler; yookassa
webhook now re-verifies payment status server-side via the API instead of
trusting the posted body (platega already had HMAC verification).

hwid: 'user["hwid_limit"] or FALLBACK' treated an explicit 0 (admin fully
blocking a user) as unset — now an explicit None check. Device count-check
and insert are now one atomic transaction (db.add_device_if_under_limit)
instead of two raceable statements.

perf: payment webhooks and _grant_paid_subscription's SSH/HTTP calls now
run via asyncio.to_thread instead of blocking the event loop; same for
bot.py's periodic_sync/reconcile_pending_payments and the manual admin
sync button. Admin endpoints (traffic/subscriptions/payments/gift-codes/
user-card) now resolve node labels from one db.list_nodes() call instead
of a fresh db.get_node() per row. revoke/reset-traffic use a direct PK
lookup instead of scanning up to 5000 rows. Dashboard now asks the API
for 8 rows instead of fetching 200 and slicing client-side.

security: mbs.db (and -wal/-shm) now chmod 600 right after creation —
it held session tokens and subscription bearer tokens world-readable
by default. Node SSH connections now pin host keys via a persisted
known_hosts file (TOFU) instead of accepting any key on every connection.
delete_node now refuses to delete a node with active subscriptions
instead of silently orphaning their xray clients.

deadcode: removed unused xray_manager.list_client_ids and admin.html's
superseded staggerReveal (rows animate via rowAttr() inline now).

Also guards gift-code redemption against a plan/node deleted after the
code was created (was an unhandled KeyError/TypeError crash).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

xray_manager.py

@@ -75,15 +75,6 @@ def remove_client(client_uuid: str):
             _reload_xray()
 
 
-def list_client_ids():
-    with _locked():
-        cfg = _load()
-        ids = set()
-        for ib in _local_inbounds(cfg):
-            ids |= {c["id"] for c in ib["settings"]["clients"]}
-        return ids
-
-
 def sync_from_db():
     import db as dbmod