feat(llm): alert when Venice credits run low

[spalen0]

Why

The Venice account ran out of credits. Every AI explanation failed, and the only trace was a log line. Safe and timelock alerts went out without an AI summary, and nothing reported it.

What

• New utils/llm/credits.py. It reads the key's balance from GET /api_keys/rate_limits because /billing/balance needs an admin key.
• Venice reports what the key can still spend: the lower of the account balance and what's left of the key's daily spend limit (currently $5). A low reading can mean the account is nearly empty or the daily cap is nearly used. The alert names both causes and shows the 00:00 UTC reset time. The inference key can't tell them apart, because /billing/balance needs an admin key.
• Called at the start of main() in protocols/safe/main.py (every 20 min) and protocols/timelock/timelock_alerts.py (hourly).
• Alerts when DIEM + bundled credits + USD (keys spend all three, in that order) is below LLM_CREDIT_ALERT_THRESHOLD_USD (default $1), or when Venice reports accessPermitted: false.
• Sends to the ops errors channel via send_error_message with notification sound on. It's labelled yearn and stored under yearn-internal, so it stays off public pages.
• Sends at most one alert every 24h while the balance stays low. A top-up clears the cooldown.
• Does nothing for providers other than Venice or when no key is set. It never raises: a failed balance read, cooldown write or alert send is logged and the monitor carries on. A failed send retries on the next run.

Testing

• tests/test_llm_credits.py: 12 cases covering the threshold, DIEM, bundled credits, blocked access, the env override, cooldown, top-up reset, fetch failure, send failure, a bad threshold value and the skip conditions.
• tests/test_safe_main.py: the quota tests that run main() now stub the check so they don't make real calls.
• Ran the balance read once against the live API and got usd=2.83. The account held $24.75 at the time, so that figure is what was left of the key's $5 daily limit.
• 4 tests in test_safe_main.py fail locally both on this branch and on unchanged main, so this PR didn't introduce them.

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