geoprobe: add ClickHouse persistence to geoprobe-target

[nikw9944]

Resolves: #3405

Summary of Changes

• Add optional ClickHouse writing to geoprobe-target — when CLICKHOUSE_ADDR is set, received LocationOffset measurements are buffered and batch-inserted into a location_offsets table with signature verification results, raw offset bytes, and structured reference arrays
• Use goose for schema migrations following the shredder pattern
• Extend TestE2E_GeoprobeDiscovery with a ClickHouse testcontainer that verifies offsets land in the table with valid signatures

RFC: RFC-16 Geolocation Verification

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	3	+312 / -4	+308
Scaffolding	2	+34 / -0	+34
Tests	2	+186 / -5	+181
Config/build	2	+92 / -72	+20
Other	1	+0 / -1	-1

Most new code is core logic (writer + migrations) and E2E verification.

Key files (click to expand)

• controlplane/telemetry/internal/geoprobe/clickhouse.go — ClickhouseConfig (env var reader), connection factory, OffsetRow conversion, buffered ClickhouseWriter with 10s flush ticker
• e2e/geoprobe_test.go — ClickHouse testcontainer helper, startGeoprobeTarget now accepts optional CH config, verifyClickhouseOffsets polls the table for valid rows
• controlplane/telemetry/internal/geoprobe/clickhouse_test.go — unit tests for OffsetRowFromLocationOffset (including TotalRttNs accumulation) and writer buffer behavior
• controlplane/telemetry/internal/geoprobe/migrations.go — goose migration runner with embedded SQL and slog-based logger
• controlplane/telemetry/cmd/geoprobe-target/main.go — optional CH init from env, writer threaded through to handleOffset
• controlplane/telemetry/db/clickhouse/migrations/20260401000001_location_offsets.sql — CREATE TABLE with MergeTree, 90-day TTL, structured reference arrays

Testing Verification

• Unit tests for OffsetRowFromLocationOffset verify field mapping, TotalRttNs accumulation across references, and raw offset hex encoding
• Unit test for ClickhouseWriter.Record confirms buffer accumulation without a real ClickHouse connection
• All existing geoprobe unit tests continue to pass (pinger, coordinator, offset serialization)
• E2E test spins up a real ClickHouse container and asserts: row count > 0, all signatures valid, total_rtt_ns > 0
• make go-lint and make go-build pass clean

[ben-dz]

If Clickhouse is unreachable, won't this hang indefinitely with the context.Background()? It should probably have some kind of timer, or just not flush the last 10s if we're closing the program?