Add subgraph production monitoring and alerting for GA

[pyropy]

Context

Per the GA scope defined in XFN-221, the backend/infra requirements include:

• Stable deployment
• Indexing monitoring
• Alerting on failures
• Target uptime: ~99.9%

Problem

Issue #87 covers subgraph exposure to pay.filecoin.cloud but doesn't address the monitoring and alerting infrastructure needed for production readiness.

Acceptance Criteria

• Set up indexing monitoring for the subgraph
• Configure alerting on indexing failures
• Document monitoring/alerting setup
• Establish baseline metrics for ~99.9% uptime target

References

• GA Scope: https://linear.app/filoz/issue/XFN-221/filecoin-pay-ga-scope
• Related issue: Setup how we're exposing our subgraph to pay.filecoin.cloud #87

[pyropy]

Possibly related to #49