Add WebSocket notifications and verifiable receipts

[collinsadi]

Summary

• Add WebSocket channel subscriptions, per-session message sequencing, heartbeat/auth expiry cleanup, and a Redis-compatible scaling adapter.
• Bridge payment/dispute domain events and analytics updates into channel-based WebSocket broadcasts.
• Add receipt Merkle proofs, search/retrieval APIs, PDF download, retention archiving, and on-chain receipt events for completed payments.

Verification

• Ran git diff --check.
• Tests not run, per delegated workflow instruction to skip tests unless CI/CD or risk requires it.

Closes #353
Closes #359

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