feat(chain-watcher): Add HTTP polling fallback when WebSocket fails

[geminimir]

Summary

Implement HTTP polling fallback for the Chain Watcher when WebSocket connection to the RPC drops. This ensures no Transfer events are missed during connection instability.

Changes

• Add ConnectionManager class to handle WebSocket connections with automatic fallback
• Detect WebSocket disconnect/error events and switch to polling mode
• Poll eth_getLogs at configurable interval (default: 2s) with block range tracking
• Resume WebSocket subscription when connection is re-established
• Implement exponential backoff for reconnection attempts (5s -> 10s -> 20s -> 60s max)
• Log mode transitions for observability
• Add ConnectionMode and TransferLog types to shared package
• Add CHAIN_WATCHER constants for polling/reconnect configuration

How to Test

1. npm ci
2. npm run typecheck
3. npm test --workspace=@stripeonchain/chain-watcher

Tests

• Tests added/updated (16 new tests for ConnectionManager)
• All tests pass (npm test)
• Lint passes (npm run lint)
• Type-check passes (npm run typecheck for chain-watcher)
• Formatting passes (npm run format:check)

Checklist

• No secrets or credentials committed
• Migrations run cleanly up and down (npm run migrate:up / npm run migrate:down)
• Docker Compose starts without errors (docker compose up)

Related Issues

Closes #14

Made with Cursor