feat: add alerts for indexer lag and source failures

[supremeproton01]

Summary

1. Lag Metrics Produced ✅
   Stellar Ledger Lag: Tracks how far behind we are from the latest Stellar ledger (via Horizon API)
   Table Ingestion Lag: Measures staleness for articles, social_posts, analytics_records, and contract_events
   Configurable Thresholds: Warning and critical severity levels per metric type

2. Alerts Configured ✅ (Log-Based for MVP)
   4 Alert Rules:
   🚨 Critical Indexer Lag (>10 minutes)
   ⚠️ Warning Indexer Lag (>2 minutes)
   ⚠️ Data Source Failures (3+ in 5 min window)
   🚨 Pipeline Falling Behind (2+ stale sources)
   Dispatch Methods: Structured JSON logging + optional Telegram/webhooks

3. Runbook Documented ✅
   ALERTING_RUNBOOK.md (1000+ lines): Complete operations guide with procedures
   ALERTING_INTEGRATION_GUIDE.md (400+ lines): Integration steps with code examples
   QUICK_REFERENCE.md: Quick lookup guide

📦 Deliverables
Core Implementation (1,100 LOC)
src/metrics/indexer_lag.py - Metrics collection
src/metrics/alerting_rules.py - Alert rules engine
src/metrics/ingestion_monitoring.py - Job orchestrator
src/metrics/init.py - Module exports

Testing (380 LOC)
tests/test_metrics_alerting.py - 30+ unit tests

Documentation & Demo
ALERTING_RUNBOOK.md - Operations guide
ALERTING_INTEGRATION_GUIDE.md - Integration guide
IMPLEMENTATION_SUMMARY.md - Delivery summary
QUICK_REFERENCE.md - Quick lookup
demo_indexer_lag_alerting.py - Interactive demo

Closes #745

Type of Change

• feat
• test

Validation

• Lint passed for affected area(s)
• Tests passed for affected area(s)
• Manual verification completed (if applicable)

Documentation

• Documentation updated (or N/A with explanation)

Checklist

• Branch name uses feat/, fix/, or docs/
• Commit messages follow Conventional Commits
• PR scope matches linked issue acceptance criteria

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[Cedarich]

@supremeproton01