feat: add suggest_cre_mappings() for automatic CRE-to-standard mapping

[Subh24ai]

Closes #585

What this does

Adds suggest_cre_mappings() to cre_main.py that automatically maps
Standard entries to CREs using cosine similarity on existing embeddings.

How it works

1. Takes a list of Standard entries (e.g. from a PCI-DSS CSV upload)
2. Generates an embedding for each section's text
3. Calls get_id_of_most_similar_cre_paginated() from PromptHandler
4. Returns high-confidence matches (>= 0.7) as mapped
5. Returns low-confidence matches as needs_review for human review

Tests

Two unit tests added — both passing.

Related

This is a foundational function for the MyOpenCRE automatic mapping
feature. Follow-up PRs will wire this into the API endpoint and UI.