perf: accept optional storage in getUsableFallbackAccounts to avoid redundant disk reads

[iceteaSA]

Summary

Adds an optional existingStorage parameter to FallbackAccountManager.getUsableFallbackAccounts(). Callers that already have storage loaded (e.g., the fetch handler which loads it at the start of every request) can pass it through to skip a redundant readFile + JSON.parse.

In the hot request path, loadAccounts() currently fires up to 3 times per request:

1. In the main fetch handler
2. Inside tryFallbackAccounts()
3. Inside getUsableFallbackAccounts()

This change eliminates #3 when the caller provides storage.

API Change

// Before
async getUsableFallbackAccounts(): Promise<OAuthAccount[]>

// After (non-breaking)
async getUsableFallbackAccounts(existingStorage?: AccountStorage | null): Promise<OAuthAccount[]>

Uses !== undefined check (not falsy) because null is a valid "no storage file" state.

Testing

All existing tests pass. No behavior change — only avoids redundant disk I/O.