geolocation: geoProbe Agent onchain target discovery

[ben-dz]

RFC Reference: rfc16 — Phase 2, item 3 (as updated by PR #2950)

Split from original version of issue #2958

Goal: Extend the geoProbe agent (Go program at controlplane/telemetry/cmd/geoprobe-agent/) to discover targets from onchain data.

Scope

Target Discovery: Query onchain GeolocationUser accounts, filter for targets where probe_pk matches this probe, extract target IPs (30s interval per RFC). Only measure targets from users with payment_status: Paid. The payment status filter is a critical security invariant — without it, unpaid users could direct probe traffic to arbitrary IPs.
Merge onchain-discovered targets with CLI-arg-based configuration
Update target list dynamically