Refactor polling services to use MultiConnectionPoller base class

[KIvanow]

Extract common multi-connection polling logic into a reusable abstract base class. Both HealthService and PrometheusService now extend MultiConnectionPoller, which provides:

• Standardized polling loop with start/stop lifecycle
• Parallel polling of all connected database instances
• Error isolation between connections
• Automatic cleanup when connections are removed
• Re-entrance guard to prevent overlapping poll cycles

This reduces code duplication and ensures consistent polling behavior across services while preserving original functionality including the 5-second startup delay for health checks and webhook triggering after metrics collection.

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Note

Medium Risk
Touches core periodic polling for health and metrics, so timing/coverage changes (especially polling disconnected connections and the new scheduler) could affect webhook firing and metric freshness if behavior diverges from the previous loops.

Overview
Refactors HealthService and PrometheusService to extend MultiConnectionPoller, removing their bespoke timer loops and centralizing multi-connection scheduling, parallel per-connection execution, overlap-guarding, and removal cleanup.

Adds shouldPollDisconnected() to the base poller and enables it for HealthService so health checks run for disconnected connections too, while preserving the 5s startup delay before polling begins. Prometheus polling now runs per-connection via pollConnection() and folds storage-based metrics updates into the same per-connection path (including cleanup on connection removal), while keeping /metrics scrapes calling updateMetrics() for connected connections.

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[KIvanow]

@cursor review

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