feat(backend): enhance error recovery for Trustline Manager

[dannyy2000]

Summary

• Replace the single global circuit breaker with a per-context registry (Map), isolating failure domains so a Horizon outage cannot block unrelated DB operations
• Add a half-open state to the circuit breaker: one probe attempt is allowed after the timeout window before the breaker fully closes, preventing premature recovery failures
• Wrap every operation in a configurable hard timeout (default 15 s) via withTimeout() to prevent runaway async calls from locking up the retry loop
• Introduce an in-memory dead-letter queue (capped at 100 entries) for terminal failures; exposes getDeadLetterQueue() and drainDeadLetterQueue() for inspection and replay
• Support pluggable fallback handlers — callers can supply an async fallback(err) that returns cached / degraded data when all retries are exhausted or the circuit is open
• Expose getCircuitBreakerMetrics() for integration with health and monitoring endpoints
• Extend error classification with dedicated timeout and auth error (401 / 403) categories
• Increase test coverage from 21 → 48 tests (all passing), adding suites for per-context isolation, half-open probing, timeout, DLQ, and fallback behaviour

Test plan

• npx vitest run src/lib/trustline-manager.test.js — 48/48 tests pass
• All pre-existing signature verification, rate limiting, and SQL optimisation tests continue to pass
• New tests cover: basic execution, all error classifications, per-context circuit breaker isolation, half-open probe success/failure, operation timeout, DLQ push/drain, and fallback handler invocation

Security notes

• No secrets or credentials touched
• Fallback handlers receive the enhanced error object (no raw DB rows or internal state leaked)
• DLQ stores only context label, error type, sanitised message, and attempt count

Closes #745
Closes #746
Closes #747
Closes #748

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