Fix 4 backend bugs: ledger caching, error format, metrics, distributed locking

[Muhammadcodes112]

• Add a thread-safe in-memory LRU cache with configurable TTL in front of the disk-backed ledger entry store in core/.
• Standardize Axum API error responses to RFC 7807 Problem Details (type, title, status, detail, instance).
• Expose Prometheus metrics for ledger indexing latency, event throughput, error counts, and job queue depth on the existing /metrics endpoint.
• Add a Redis-backed leader lease lock so only one Core instance performs ledger fee collection at a time, avoiding duplicate processing across multiple instances.

Manually traced each changed code path (cache read/write flow, error response construction, metrics registration/recording, and leader-lock acquire/renew logic) since the repo has no working automated test/build verification available in this environment.

Closes #560
Closes #561
Closes #563
Closes #564

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