Document funding account operational requirements

[phertyameen]

Location

docs/deployment.md

Problem

No confirmed documentation covers the operational requirements around FUNDING_ACCOUNT_SECRET (minimum balance, monitoring, what happens if it runs dry), relevant for whoever operates this in production.

Acceptance Criteria

• Covers minimum recommended balance with rationale
• Cross-references the low-balance alerting issue (section F) as the automated safeguard
• Includes guidance on securing the secret itself, not just operating the balance