feat: shared HTTP client pool with per-host budgets

[Killerjunior]

Description

Introduces a shared, resilient HTTP client pool in internal/httpx with per-host connection budgets, per-host circuit breakers, and periodic DNS refresh to avoid stale A-record connection storms during failovers.

Changes Included

1. Core HTTP Client Pool (internal/httpx)

• Per-Host Connection Budgeting: Enforces strict MaxConnsPerHost and MaxIdleConnsPerHost limits per remote target host to prevent noisy neighbors from starving outbound slots.
• Per-Host Circuit Breaker: Integrates gobreaker state per host to fast-fail requests with ErrCircuitOpen when upstream failure thresholds are breached.
• DNS Refresh & TTL Support: Uses net.Resolver with a pluggable Clock interface. Automatically re-resolves target hosts when DNSTTL expires (or on every dial when DNSTTL = 0). Recycles idle connections whenever host address changes occur.
• Prometheus Metrics: Instrument http_client_conn_reuse_ratio gauge labeled by host to monitor connection reuse efficiency.

2. Outbox & Integrations Wiring

• Outbox System: Wired outbox HTTPPublisher and SlackPublisher through httpx.Pool via PooledHTTPClient and PooledSlackClient.
• Service Configuration: Updated outbox.NewService to accept optional custom HTTPPool while defaulting to the package-level shared pool.
• Integrations: Integrated PagerDuty client (internal/integrations/pagerduty) to back outbound event delivery using the shared httpx.Pool.

Verification

• Added test suite in internal/httpx/client_test.go covering:
  • Connection reuse ratio metric tracking
  • Per-host max connection limits under high concurrency
  • Circuit breaker trips on consecutive transport failures
  • DNS TTL zero refresh on every call vs cached TTL behavior
  • Automatic recycling of idle connections when DNS addresses change
  • Resolver failure fallbacks and malformed address handling

Closes #687

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[Killerjunior]

Review please.