[$25 BOUNTY] [Rust] Add graceful shutdown config docs and parser tests

[Saiaaax]

Summary

Adds parser tests and operational documentation for the TOT_SHUTDOWN_GRACE_SECS shutdown grace-period setting.

Closes #72

Changes

• New file: backend/src/shutdown_grace.rs — Grace-period parser module with:

  • GracePeriod struct with resolved Duration and GraceSource enum
  • parse_grace_period() reads TOT_SHUTDOWN_GRACE_SECS env var
  • parse_grace_period_from() testable entry-point accepting optional string
  • Constants: DEFAULT_GRACE_SECS (30), MIN_GRACE_SECS (1), MAX_GRACE_SECS (300)
  • 10 unit tests covering: unset/default, valid in range, valid at min/max boundary, zero clamped, negative clamped, too-large clamped, non-numeric fallback, whitespace trimming, float-string fallback

• Updated: backend/src/lib.rs — Added pub mod shutdown_grace;

• Updated: docs/OPERATIONS.md — Added "Graceful Shutdown Configuration" section documenting the env var, default, valid range, behavior table, how to run tests, and Kubernetes integration example

Testing

cargo test shutdown_grace

Tests cover all acceptance criteria:

• Unset → default (30s)
• Valid integer → parsed value
• Zero → clamped to 1s
• Negative → clamped to 1s
• Too-large (>300) → clamped to 300s
• Non-numeric → fallback to default

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-db991709.logd