fix(settings): validate DUCKDB_THREADS instead of silently producing NaN

[pedrofrxncx]

This is the same env-var validation vein as #5473/#5476/#5463/#5379 etc: DUCKDB_THREADS was the last remaining hand-rolled Number(...) in resolve-config.ts with no validation (grepped for Number( in the file — the other two hits are already-guarded helpers). A malformed value ("abc", "-1", "1.5") silently produced NaN in Settings.duckdbThreads, which only surfaces later as an opaque failure wherever the embedded DuckDB monitoring engine consumes it, instead of failing fast at startup with a clear message.

Note: an older open PR (#5130) touched this exact env var, but against the pre-rename apps/mesh/... path, which no longer exists in this repo (renamed to apps/api months ago) — it can't apply as-is. This PR re-does the fix at the current path.

Fix: route DUCKDB_THREADS through the same toPositiveIntegerOrUndefined helper already used for CLICKHOUSE_MAX_MEMORY_USAGE and other optional numeric env vars in this file, so an invalid value throws "DUCKDB_THREADS must be a positive integer" at boot instead of silently becoming NaN.

Failure scenario: an operator sets DUCKDB_THREADS=4.5 (or any non-integer) in their deployment env — before this fix, Settings.duckdbThreads becomes NaN and the process boots successfully, only breaking later when the DuckDB monitoring engine reads it. After this fix, resolveConfig throws immediately at startup.

Test: added a resolveConfig duckdb threads describe block in resolve-config.test.ts mirroring the existing clickhouse max memory usage test shape — covers unset (undefined default), a valid value, and it.each over invalid values ("abc", "0", "-1", "1.5", "Infinity").

Reviewer command: bun test apps/api/src/settings/resolve-config.test.ts

Locally verified: bun run fmt, cd apps/api && bunx tsc --noEmit, bun test apps/api/src/settings/resolve-config.test.ts (87 pass), bunx oxlint on both changed files (0 warnings/errors). Full CI validates the rest.

---

Summary by cubic

Validate the DUCKDB_THREADS env var to reject non-integer or non-positive values, so config fails fast with a clear error instead of propagating invalid numbers. Adds targeted tests and follows the existing numeric env var validation pattern.

• Bug Fixes
  • Parse DUCKDB_THREADS via toPositiveIntegerOrUndefined in resolve-config, defaulting to undefined when unset and accepting integers (e.g., "4").
  • Throw "DUCKDB_THREADS must be a positive integer" at startup for invalid values like "abc", "0", "-1", "1.5", "Infinity".
  • Added tests in apps/api/src/settings/resolve-config.test.ts covering unset, valid, and invalid cases.

^{Written for commit 2e92a9d. Summary will update on new commits.}