[RESILIENCE] Circuit breaker pattern — prevent cascading failures when dependencies are slow/unavailable

[ElioNeto]

Description

When external dependencies (disk, network, storage) are slow or failing, ApexStore should fail fast instead of queuing requests that will time out anyway.

Implementation

1. Add CircuitBreaker wrapper around critical I/O paths (WAL fsync, SSTable reads, compaction)
2. Configurable thresholds: failure count, timeout window, half-open retry interval
3. States: CLOSED (normal), OPEN (fail fast), HALF_OPEN (test recovery)
4. Expose circuit state in /metrics endpoint
5. Log state transitions for alerting

Configuration

resilience:
  circuit_breaker:
    wal_fsync:
      failure_threshold: 5
      timeout_ms: 10000
      half_open_after_ms: 30000
    compaction:
      failure_threshold: 3
      timeout_ms: 60000

Labels

[ElioNeto]

Implemented in commit 0871d91.