fix: harden auth and investment synchronization

Resolve A01-A10 and C01-C07 from the backend audit.
Preserve auth failures, forced candle reconciliation and forecast freshness.
Validate MOEX responses and nested HTTP errors; remove unused legacy paths.

Verified: 209 unit tests, 262 SQLite e2e tests on Linux, OpenAPI and durability.
Record the remaining macOS native SQLite abort in the audit.

src/integrations/tbank/tbank-account.policy.ts

@@ -39,17 +39,6 @@ export function supportedTbankAccounts(
   });
 }
 
-export function accessibleTbankAccount(
-  accounts: TbankAccount[],
-  accountId: string,
-): TbankAccount | null {
-  return (
-    accounts.find(
-      (account) => account.id === accountId && account.type in accountTypes,
-    ) ?? null
-  );
-}
-
 function openedOn(seconds: string | number | undefined): string | null {
   if (seconds === undefined) return null;
   const date = new Date(Number(seconds) * MILLISECONDS_PER_SECOND);