telemetry: consolidate clickhouse schema migrations

[snormore]

Summary of Changes

• Introduce telemetry/migrations package: a shared goose migration runner (embedded SQL, slog adapter, RunMigrations func) used by all telemetry services that write to ClickHouse
• Consolidate flow-enricher and gnmi-writer ClickHouse schemas into versioned goose migrations: flows (with enrichment columns), device_ifindex, and all 7 gnmi-writer tables
• Add CLICKHOUSE_RUN_MIGRATIONS env var / --clickhouse-run-migrations flag to flow-enricher and gnmi-writer for on-startup migration execution
• Update both integration tests to apply schemas via RunMigrations — tests now run against the same migrations as production
• Add setup.sql for one-time admin setup of telemetry_mainnet_beta, telemetry_testnet, and telemetry_devnet databases with writer users
• Remove per-service clickhouse/ schema directories and the flow_schema_migrations.yml CI workflow (migration CI moves to the infra repo)

Notes

flows table engine: The migration specifies ENGINE = MergeTree() rather than SharedMergeTree. ClickHouse Cloud automatically promotes MergeTree (and its variants) to the equivalent SharedMergeTree engine transparently — specifying SharedMergeTree explicitly is unnecessary and breaks single-node deployments such as the test containers.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	2	+352 / -1	+351
Scaffolding	12	+58 / -222	-164
Tests	2	+36 / -18	+18
Fixtures	2	+9 / -66	-57
Config/build	2	+25 / -85	-60
Generated	1	+53 / -0	+53
Total	21	+526 / -392	+134

Most net-new lines are goose migration SQL; the rest is deletion of old scattered schema files.

Key files (click to expand)

• telemetry/migrations/20250303200213_gnmi_init.sql — new goose migration containing all 7 gnmi-writer tables and their _latest views
• telemetry/migrations/migrations.go — RunMigrations func: opens a ClickHouse database/sql connection, sets the embedded FS, runs goose up; slog adapter for goose logging
• telemetry/migrations/20250303200212_flows_init.sql — moved from flow-enricher; adds enrichment columns (src_device_code, dst_device_code, src_location, dst_location, src_exchange, dst_exchange)
• telemetry/migrations/20250303200214_device_ifindex_init.sql — new migration for the device_ifindex lookup table read by flow-enricher
• telemetry/gnmi-writer/cmd/gnmi-writer/main.go — adds ClickhouseRunMigrations to Config, wired from CLICKHOUSE_RUN_MIGRATIONS / --clickhouse-run-migrations
• telemetry/flow-enricher/cmd/flow-enricher/main.go — same CLICKHOUSE_RUN_MIGRATIONS wiring in the non-stdout output path
• telemetry/migrations/setup.sql — one-time manual setup: creates three databases and per-database writer users

Testing Verification

• go build and go vet pass on all affected packages
• gnmi-writer integration test updated to apply schemas via RunMigrations against a testcontainer
• flow-enricher integration test updated to apply schemas via RunMigrations and seed device_ifindex from the insert fixture; test now writes to flows (production table name) instead of flows_integration