Set a monthly spend cap on the Anthropic API key

[Salmatcre8]

Summary

Set a hard monthly spend cap on the Anthropic API key that powers the AI agent. Every agent
message costs Claude tokens; without a ceiling, abuse or a bug could run up a surprise bill.

The rate limiter on /api/agent (separate issue) slows abuse — this spend cap is the actual
guarantee against a runaway bill. We want both.

⚠️ Owner-only task. Requires access to the Anthropic billing console. No code change.

Tasks

• Go to console.anthropic.com → Billing / Usage limits.
• Set a monthly usage limit we're comfortable with for launch (start conservative — it can be
  raised anytime).
• Confirm email alerts are on so we get warned before the cap is hit.

Acceptance criteria

• A monthly spend limit is visibly set in the Anthropic console.
• Usage/billing alert emails are enabled.

Notes

• Pair with the /api/agent rate-limit issue. Rate limit = first line of defense; spend cap =
  the hard ceiling behind it.
• This is our single biggest variable cost — worth watching in the first weeks of traffic.