feat: resilient Pulse registration with ServiceAPIKey

[davidfic]

Summary

Switches from fire-and-forget `sdk.New()` to `sdk.NewWithRetry()` for Pulse registration. Haul now retries registration with exponential backoff (~2 min), then continues standalone if Pulse is unavailable. Also passes Haul's auto-generated API key as `ServiceAPIKey` so Pilot/Prism can discover and authenticate with Haul automatically via Pulse.

What changed

• integration.go — `New()` accepts a `serviceAPIKey` parameter, uses `sdk.NewWithRetry()` instead of `sdk.New()`, passes `ServiceAPIKey` in the SDK config
• main.go — Registration is no longer in a fire-and-forget goroutine. Blocks during the retry window, then continues regardless. Passes `cfg.Auth.APIKey.Value()` as the service API key
• go.mod — Pins pulse SDK to v0.2.0 (adds `NewWithRetry`, `ServiceAPIKey`, resilient heartbeat)

End-to-end flow

1. Haul starts, tries to register with Pulse for ~2 min
2. If Pulse responds: registered with API key stored in Pulse's services table. Pulse auto-creates a download-client entry with Haul's URL and API key. Pilot/Prism pick it up via their existing `SyncDownloadClients()` loop.
3. If Pulse doesn't respond: Haul logs a warning and runs standalone. Users configure download clients manually via the UI.

Test plan

• Build + test pass locally
• CI green
• `docker compose up` with Pulse available → Haul appears as download client in Pilot/Prism within 30s
• `docker compose up` without Pulse → Haul starts standalone after ~2 min of retries