feat: add circuit breaker for Stellar RPC failures

[GautamKumarOffical]

Implements a circuit breaker pattern to prevent cascading failures when the Stellar RPC node is experiencing issues.

Changes

New file: apps/backend/src/common/circuit-breaker.ts

• Three states: CLOSED (normal), OPEN (fail fast), HALF_OPEN (testing recovery)
• Circuit opens after 5 consecutive failures within 60 seconds
• When open, requests fail immediately without calling RPC (fail fast)
• After 30 seconds, transitions to HALF_OPEN and allows 1 test request
• If test succeeds → CLOSED. If fails → back to OPEN
• All state changes are logged with timestamps

New file: apps/backend/src/common/circuit-breaker.spec.ts

• 10 unit tests covering all state transitions, window resets, force-reset, and logging

Modified: apps/backend/src/submissions/submissions.service.ts

• Integrated circuit breaker into callContractApprove
• Fail-fast when circuit is OPEN (returns 503 to caller)
• Records success/failure after each RPC attempt cycle

Modified: apps/backend/src/metrics/metrics.service.ts

• Added Prometheus gauge: stellar_bounty_circuit_breaker_state (0=closed, 1=open, 2=half_open)
• Added Prometheus counter: stellar_bounty_circuit_breaker_state_changes_total

Configuration (env vars)

• CIRCUIT_BREAKER_FAILURE_THRESHOLD (default: 5)
• CIRCUIT_BREAKER_FAILURE_WINDOW_MS (default: 60000)
• CIRCUIT_BREAKER_OPEN_TIMEOUT_MS (default: 30000)

How it works

1. RPC calls flow through the circuit breaker
2. Each failed RPC cycle (all URLs exhausted) increments the failure counter
3. After 5 failures within 60s → circuit OPENS → immediate rejection
4. After 30s → HALF_OPEN → one test RPC call
5. Test succeeds → CLOSED. Test fails → OPEN again

Closes #202

[GBOYEE]

I'd like to work on this.

Approach:

• I'll add the CI config and test it locally before pushing
• Verify with existing tests + add new ones if needed

Estimated effort: ~1-2 hours. PR incoming shortly.
Closes #293

[Xuccessor]

Thanks for the PR. Two things: CI has a failing Backend check on this branch, and issue #202 is assigned to @teethaking — only the assigned contributor's PR is being merged here. Worth coordinating in the issue thread.

[Xuccessor]

Closing this — #202 is assigned to @teethaking. Thanks for the work!