Use persistent Electrum subscriptions

[StellarStoic]

Summary

• keep one reconnecting Electrum JSON-RPC session instead of opening a TCP connection for every request
• subscribe to each watched script hash before requesting its history or balance
• cache unchanged snapshots and refresh only after Electrs sends a subscription status change
• wake the scanner immediately on activity while retaining the 30-second poll as a reconnection and safety fallback
• automatically resubscribe and reconcile cached state after Electrs restarts

This removes the repeated get_balance/get_history called for unsubscribed scripthash warnings emitted by recent Electrs versions and substantially reduces idle local-node traffic for large xpub and bulk-address watches.

Verification

• go test ./...
• go vet ./...
• go test -race ./internal/electrum ./internal/app
• new fake-Electrum tests verify subscribe-before-query ordering, unchanged-snapshot caching, notification-driven refresh, and reconnect/resubscribe behavior
• subscription tests passed 50 consecutive runs
• production Dockerfile build passed