feat(providers): queue update events through bounded per-provider buffers

Update events submitted by the triggers (poll, webhook, pubsub, approvals)
are now queued in a bounded event buffer per provider instead of being
processed inline. The buffer applies backpressure to producers when it
saturates, and its capacity is configurable via the EVENT_BUFFER_SIZE
environment variable (default 512) and the chart's eventBufferSize
value. Adds internal/concurrent helpers for the worker pools, a
kubernetes_provider_event_buffer_backpressure_total metric, and
unit tests for the buffer, backpressure, and concurrency behavior.

Spec-Ref: helix-specs@d72d7938:000397_investigate-and-fix-with