feat(api): moneypath resilience + webhook security hardening

[Johnalex-hub]

Summary

Covers 4 issues. Work in progress — marking ready for review once all 4 are implemented.

• feat(api): outbox delivery latency metrics and alerting thresholds #1311 outbox delivery latency metrics — done (time-to-first-attempt, time-to-delivery per event type)
• feat(api): moneypath circuit breaker for degraded downstream chain RPC #1314 moneypath circuit breaker configurable trip thresholds — done. Note: a circuit breaker already exists for Soroban RPC/Horizon (nester#1087, internal/breaker) with configurable thresholds, wired at startup, surfaced at 503+Retry-After. This PR adds independent per-upstream threshold overrides (CIRCUIT_BREAKER_SOROBAN_RPC_* / CIRCUIT_BREAKER_HORIZON_*) on top of the existing shared policy.
• feat(api): webhook signature verification for inbound protocol callbacks #1312 webhook signature verification for inbound protocol callbacks — pending
• feat(api): webhook replay protection using nonce/timestamp window #1313 webhook replay protection (nonce/timestamp window) — pending

Test plan

• go build ./... passes
• Unit tests for new config override paths
• Unit tests for outbox latency metrics
• feat(api): webhook signature verification for inbound protocol callbacks #1312/feat(api): webhook replay protection using nonce/timestamp window #1313 implementation + tests

Closes #1311
Closes #1314
Closes #1312
Closes #1313

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📒 Files selected for processing (4)

• apps/api/cmd/api/main.go
• apps/api/internal/config/config.go
• apps/api/internal/domain/outbox/metrics.go
• apps/api/internal/domain/outbox/relay.go

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