feat: implement caching for user admission check

[Mohit-Davar]

Description

This PR implements the TODO: use cache in EventMessageHandler by introducing a Redis-based caching layer for user admission status.

Key Implementation Details

• State-Aware Caching

  • Uses getKey to differentiate:
    • 1 → Admitted users
    • 0 → Blocked / insufficient balance
    • null → Cache miss

• TTL Strategy

  • Admitted users: 300s (5 minutes)
  • Blocked / low-balance users: 60s (1 minute)
  • Prevents database hammering while keeping data reasonably fresh

• Atomicity & Resilience

  • Uses a non-blocking cacheSet helper
  • Redis operations are async and do not impact event ingestion latency

• Testing

  • Updated event-message-handler.spec.ts with mock cache behavior

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Related Issue

Resolves the caching TODO in:
src/handlers/event-message-handler.ts
#447

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Motivation and Context

On high-traffic relays, performing database checks for every event introduces significant overhead.
This change:

• Reduces database load
• Improves event processing latency
• Enhances scalability under high throughput

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How Has This Been Tested?

• Unit Tests
  • Updated test/unit/handlers/event-message-handler.spec.ts

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Screenshots

N/A

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Types of Changes

• Non-functional change (docs, style, minor refactor)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to change)

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Checklist

• Code follows project style guidelines
• Documentation updated (if required)
• CONTRIBUTING guidelines reviewed
• Tests added/updated
• All tests passing

[Copilot]

Pull request overview

Implements Redis-backed caching for the EventMessageHandler.isUserAdmitted check to reduce repeated database lookups during high-throughput event ingestion.

Changes:

• Injected an ICacheAdapter into EventMessageHandler and added cache read/write logic for admission decisions.
• Introduced a Redis-backed cache singleton in message-handler-factory for reuse across handlers.
• Updated unit test constructions of EventMessageHandler to pass a cache adapter stub.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/handlers/event-message-handler.ts	Adds Redis cache read/write paths and a helper for non-blocking cache writes in isUserAdmitted.
src/factories/message-handler-factory.ts	Creates/reuses a Redis cache adapter instance and injects it into EventMessageHandler.
test/unit/handlers/event-message-handler.spec.ts	Updates handler instantiation to include the new cache dependency.

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