feat: add dry-run query param to build endpoint

[Scr4tch587]

What

Expose dry runs over HTTP via ?dry-run=true on POST /build.

How

• new dry-run query param (default false)
• real build still returns {"status": "ok"}
• dry run returns {dataset_name, dataset_version, dry_run, rows} (rows shaped like GET /data)
• failures surface the same 400/422/500 semantics as a real build

Testing

• route tests: dry-run returns rows, default is not dry-run, empty rows, 422 passthrough
• integration: DB untouched, builders actually run, dependency chain + lookback, store parity

[github-actions]

• main
  • feat: add Store abstraction for build data access #352
    • feat: add dry-run flag to build_dataset #353
      • feat: add dry-run query param to build endpoint 👈
        • docs: document dry-run builds in backend spec #355