[$25 BOUNTY] [Go] Add market gateway readiness drain flag

[Saiaaax]

Summary

Adds a readiness drain flag so operators can mark the market gateway as draining. When draining, /health/ready returns 503 with {"status":"draining"} while preserving normal healthy output otherwise.

Closes #69

Changes

• New: market/gateway/readiness.go — ReadinessProbe with Ready/Draining states, concurrent-safe via sync.RWMutex
• New: market/gateway/readiness_test.go — 6 tests:
  • TestNewReadinessProbeStartsReady — initial state is Ready
  • TestSetDrainingTransitionsState — transition Ready → Draining
  • TestSetReadyTransitionsBack — transition Draining → Ready
  • TestReadinessStateString — String() labels for states
  • TestDrainingGatewayReturnsNotReady — gateway integration
  • TestConcurrentReadinessAccess — concurrent read/write safety
• Updated: market/gateway/api.go — Added readiness field, updated handleReadiness() to check drain state, added Drain() and Readiness() methods
• Updated: docs/OPERATIONS.md — Added "Market Gateway Readiness Drain" section

Testing

cd market && go test ./gateway/ -run Readiness -v

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging

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Payout address: 0x5253757322092903C2F0a8EfEed592344216CC7E
Diagnostic artifact reference: diagnostic/build-a2d43c2b.logd