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.

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[greptile-apps]

Greptile Summary

The PR adds a validated CLICKHOUSE_MAX_OPEN_CONNECTIONS setting, passes it to the ClickHouse client when configured, and documents per-instance pool sizing.

• Preserves the client’s default connection limit when the setting is unset.
• Adds runtime and database-preflight validation for positive safe integers.
• Adds tests for configured, unset, and invalid values.

Confidence Score: 4/5

The PR should not merge until ClickHouse-specific validation is gated so PostgreSQL-only deployments cannot be broken by an irrelevant environment value.

The new module-scope and preflight checks reject CLICKHOUSE_MAX_OPEN_CONNECTIONS before determining whether ClickHouse is enabled, making an unused setting capable of breaking route loading, builds, and container startup.

Files Needing Attention: src/lib/clickhouse.ts, scripts/check-db.js

Important Files Changed

Filename	Overview
src/lib/clickhouse.ts	Adds the client pool option, but validates it at module load even when ClickHouse is disabled.
scripts/check-db.js	Adds matching preflight validation before both ClickHouse applicability and the SKIP_DB_CHECK bypass.
src/lib/clickhouse.test.ts	Covers valid, unset, and malformed values but not the disabled-ClickHouse or SKIP_DB_CHECK cases.
README.md	Documents the option, client default, and aggregate pool sizing across replicas.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  Env[Environment] --> URL{CLICKHOUSE_URL set?}
  Env --> Limit[Parse CLICKHOUSE_MAX_OPEN_CONNECTIONS]
  Limit -->|Invalid| Fail[Throw or exit]
  URL -->|Yes| CH[Create ClickHouse client]
  URL -->|No| PG[Use PostgreSQL]
  Limit -->|Valid or unset| CH
  Limit -. currently evaluated even here .-> PG

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