feat: make ClickHouse connection limit configurable

[arthurbragaa]

Summary

• add optional CLICKHOUSE_MAX_OPEN_CONNECTIONS configuration
• validate positive integers during database preflight
• preserve the ClickHouse client default when unset
• document the default and per-instance pool sizing

Closes #4491.

Verification

• pnpm test — 95 files, 748 tests
• pnpm exec tsc --noEmit
• pnpm build
• changed-file Biome lint
• production Docker image build
• Docker Compose integration: default peaked at 10 connections; configured 25 peaked at 25 with 50/50 successful /api/send requests; invalid 0 exited before health startup

Full pnpm lint remains blocked by pre-existing diagnostics on dev; the changed files pass Biome.

AI assistance disclosure: This contribution was implemented and tested with AI assistance and reviewed by Arthur Braga Alfredo.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}

[vercel]

@arthurbragaa is attempting to deploy a commit to the Umami Software Team on Vercel.

A member of the Team first needs to authorize it.

[greptile-apps]

Greptile Summary

The PR makes the ClickHouse client connection limit configurable while retaining the client default when unset.

• Adds positive-integer validation for enabled ClickHouse deployments.
• Skips this validation when ClickHouse is disabled or database preflight is explicitly skipped.
• Adds focused client-option tests and deployment sizing documentation.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/lib/clickhouse.ts	Gates connection-limit parsing on ClickHouse being enabled and conditionally forwards the validated value to the client.
scripts/check-db.js	Validates the optional limit only after the preflight skip and only for enabled ClickHouse deployments.
src/lib/clickhouse.test.ts	Covers defaults, configured limits, invalid values, and disabled ClickHouse behavior.
README.md	Documents the option, current client default, and per-instance pool sizing.

_{Reviews (2): Last reviewed commit: "fix: ignore ClickHouse limit when disabl..." | Re-trigger Greptile}

[mikecao]

@greptile