Replace real-derived regression fixtures with deterministic synthetic data

[tuoro]

Why

The previous regression samples were derived from real invoices and tickets. This change removes those samples from the current tree and closes the production capture/export path that could recreate or expose them.

What changed

• Replace the old fixture set with one fixed, deterministic synthetic generator covering five regression scenarios.
• Remove regression fixture export, import, sync, force, and redaction flows, including their production backend and frontend endpoints.
• Tighten the legacy-data migration to require all three identifying columns and fail closed at startup when cleanup cannot be completed.
• Keep regression fixture data out of the final container image.
• Enforce LF line endings for generated fixture files on Windows.

Impact

• The application no longer exposes UI or API flows for capturing, managing, importing, or exporting regression samples.
• The migration targets only rows matching the complete legacy regression schema; normal invoice, payment, OCR, and email records are not selected by that cleanup condition.

Validation

• Canonical fixture validation and the complete Go test suite
• go vet, go mod verify, and a temporary Go build
• Frontend lint, 10 test files / 58 tests, and production build
• Base and production Docker Compose configuration rendering
• Clean-checkout verification with Windows core.autocrlf=true
• Independent release review: no P0, P1, or P2 findings

CI follow-up

The PR remains a draft while CI verifies the exact head commit, including CGO-enabled SQLite migration behavior and the actual final container image.

Scope boundary

This PR changes the current repository tree only. Rewriting Git history or removing historical tags, GitHub Releases, published container images, and Actions caches requires a separate, explicitly approved cleanup.

Reviewed head: 8344de9cc29706620bff168063876d5c0741d44a