feat: implement traffic mirroring for canary deployments (#639)

[eleven-smg]

What this PR does

Adds traffic mirroring support so new canary versions can be tested
against live traffic without affecting current clients.

Changes

• Added TrafficMirrorMiddleware that duplicates incoming requests
  to a configurable canary host in the background
• Added CanaryMetricsService that tracks mirrored request outcomes
  and logs promote or rollback signals based on success rate thresholds
• Added CanaryModule to wire the middleware and service together
  and apply mirroring globally
• Registered CanaryModule in app.module.ts

How it works

• Set CANARY_MIRROR_ENABLED=true, CANARY_HOST, and CANARY_PORT
  in your environment to enable mirroring
• Mirrored requests run in the background and never block the
  original response
• The metrics service automatically logs a promote signal when
  success rate reaches 95% and a rollback signal when it drops below 70%

Closes #639

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