fix: remove Thread.sleep chaos injection from CreditCardService; add latency gate and regression test

[Copilot]

Random 0–9s Thread.sleep on every GET /api/cards request caused avg response times of 1–7.6s (SLA: <200ms), triggering Azure Monitor alert alert-latency-sre-three-rivers.

Changes

• CreditCardService.java — Remove chaos injection:

  - try { Thread.sleep((long)(Math.random() * 9000)); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }

• ci.yml — Fail CI if Thread.sleep appears in backend/src/main/, preventing recurrence:

  - name: Check for Thread.sleep in production code
    run: |
      if grep -rn "Thread\.sleep" backend/src/main/; then
        echo "ERROR: Thread.sleep found in production source code. Remove it before merging."
        exit 1
      fi

• CreditCardServiceTest.java — Latency regression test asserting getAllCreditCards() completes in <500ms using System.nanoTime().

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