Add a field catalog and finish the template PDF flow

[chetanr25]

Add a field catalog and finish the template PDF flow

Why

Setting up a template meant mapping its fields to the incident contract, but there was no way to search the contract for the right field, and the PDF upload flow had no step to review a detection draft or get the source file back. The template schema file also needed room for a few new concepts: per-field sources, layout, and the mapping suggestions.

What changed

• Added GET /schema/fields, a search endpoint over the flattened incident contract (path, type, section, aliases, PII flag), generated from the contract itself so it never drifts out of sync. This backs the mapping picker in the template editor and the auto-suggestions shown after field detection.
• Added GET /templates/{template_id}/pdf to fetch a template's original source PDF.
• Added GET /templates/pdf/{upload_id} to review a field-detection draft before registering it as a template.
• Added the template_field_detection job type to the shared job schemas.
• Expanded template.yaml in place, keeping its filename so the existing references from the template paths stay valid, to carry FieldSource per field, the field layout, and the mapping suggestions the field catalog produces.

Reference

Discussion: #609.
Related issue: #621