fix(backend): emit LF-only OpenAPI spec to avoid phantom Windows CRLF diffs

[simonpeters298]

Closes #1374

Problem

backend/openapi.json is reported as modified (M) after npm run generate:openapi on Windows even though the file is byte-identical to HEAD — only CRLF/LF normalization differs. This produces phantom working-tree changes that make it hard to tell whether the spec actually drifted.

Root cause

backend/scripts/generate-openapi.ts preserved whatever line ending the existing output file happened to have:

const eol = fs.existsSync(outputPath) && fs.readFileSync(outputPath, 'utf8').includes('\r\n') ? '\r\n' : '\n';
fs.writeFileSync(outputPath, content.replace(/\n/g, eol), 'utf8');

Once openapi.json had CRLF in the working tree (a Windows editor, or a legacy checkout from before .gitattributes existed), every regeneration rewrote it as CRLF. Git then flagged the file as modified while the diff was empty:

CRLF working copy : " M backend/openapi.json"   (git diff -> 0 lines)
LF   working copy : clean

Fix

1. backend/scripts/generate-openapi.ts — always write LF. Output is now platform-independent and matches the repository's .gitattributes (* text=auto eol=lf). Removed the now-unused os import.
2. .gitattributes — pin generated artifacts with an explicit text eol=lf (backend/openapi.json, backend/schema-snapshots/*.json, docs/schemas/webhooks/*.json) so they are always treated as text and checked out as LF.
3. scripts/validate-generated-artifacts.ts + test — new guard that fails if any committed generated artifact contains CRLF. Wired as pnpm validate:line-endings / pnpm test:validate-line-endings.

Verification

• vitest run scripts/validate-generated-artifacts.test.ts -> 4/4 passing (8/8 together with the existing image-size tests).
• git check-attr now reports text: set / eol: lf for the generated artifacts.
• backend/openapi.json itself is unchanged — it was already LF, so there is zero artifact churn.
• The phantom-diff condition is reproduced and confirmed resolved: a CRLF working copy shows M with an empty diff, whereas an LF working copy is clean, and the generator now always writes LF.

Notes

• Only generate-openapi.ts had the EOL-preserving heuristic; the schema-snapshot and webhook JSON-schema generators already write LF.
• This addresses line endings introduced or kept by regeneration. If an editor introduces CRLF between runs, git add --renormalize remains the escape hatch, which is inherent Git behavior.

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