feat(lake): bound ClickHouse connection pool (SC-9)

[TerrifiedBug]

SC-9 — ClickHouse connection pooling

Problem

getLakeClient() (src/server/services/lake/clickhouse.ts) built the cached @clickhouse/client client via createClient() with no pool configuration, so under load the lake opened unbounded connections.

Change

• createClient() now receives bounded connection settings, sourced from the centralized env module (mirrors the prisma.ts → env.DATABASE_POOL_MAX convention):
  • max_open_connections ← VF_LAKE_CH_POOL_MAX (conservative default 10)
  • request_timeout ← VF_LAKE_CH_REQUEST_TIMEOUT_MS (default 30000ms) — fail slow lake requests instead of hanging a held connection
  • keep_alive: { enabled: true } — set explicitly (it is the @clickhouse/client@1.x default) to document intent / socket reuse
• New env vars added to src/lib/env.ts following the existing DATABASE_POOL_MAX / DATABASE_CONNECTION_TIMEOUT_MS pattern (z.coerce.number().int().positive().default(...)).
• Option names verified against @clickhouse/client@1.20.0 (not invented): max_open_connections, request_timeout, keep_alive.
• The pool is per-client, so the single cached-client pattern (globalForLake.__vfLakeClient) is kept unchanged — the pool is the process-wide ceiling.

Acceptance

The lake ClickHouse client is created with a bounded connection pool. ✅

Tests

src/server/services/lake/__tests__/clickhouse-pool.test.ts (new):

• getLakeClient() calls createClient with max_open_connections from env — default (10) and overridden (25).
• request_timeout + keep_alive: { enabled: true } are passed.
• Client is cached — createClient runs once across two getLakeClient() calls.

@clickhouse/client is mocked (not installed in OSS dev); @/lib/env is mocked with a mutable hoisted object so default + override are exercised under static imports (no dynamic import / module reset, per repo rules).

pnpm exec vitest run src/server/services/lake/__tests__/clickhouse-pool.test.ts   # 3 passed
pnpm exec vitest run src/server/services/lake/__tests__/clickhouse.test.ts        # 11 passed (unaffected)
pnpm exec vitest run src/lib/__tests__/env.test.ts                                # 21 passed (unaffected)

Notes

• No migration.
• Filtered typecheck: clean except the known, pre-existing @clickhouse/client module-not-found on the existing import (package absent in OSS dev) — to be ignored per the local-gap convention.
• ClickHouse/lake runtime paths aren't locally/visually verifiable; unit tests + types are the bar.
• License boundary respected: no @cloud/* imports.