feat: add POST /rest/v1/suggest_cre_mappings API endpoint

[Subh24ai]

Closes #585 (partial)

What this does

Exposes suggest_cre_mappings() as a REST API endpoint, making the
AI-powered CRE mapping feature accessible from the MyOpenCRE frontend.

Endpoint

POST /rest/v1/suggest_cre_mappings

Request: multipart/form-data with cre_csv file
Response:

{
  "mapped": [
    {
      "standard": {"name": "PCI-DSS", "section": "..."},
      "suggested_cre_id": "cre-db-id-123",
      "confidence": 0.85
    }
  ],
  "needs_review": [...]
}

How it works

1. Accepts CSV upload of standard entries
2. Parses using existing parse_export_format()
3. Calls suggest_cre_mappings() from PR feat: add suggest_cre_mappings() for automatic CRE-to-standard mapping #753
4. Returns high-confidence matches and low-confidence ones for human review

Tests

19 tests passing including new endpoint test.

Related

• Builds on PR feat: add suggest_cre_mappings() for automatic CRE-to-standard mapping #753 (suggest_cre_mappings backend function)
• Part of MyOpenCRE automatic mapping feature (MyOpenCRE: allow for mapping CREs to a standard automatically #585)