SEC: Escape spreadsheet formulas in CSV exports

[rootverdict]

Summary

• neutralize spreadsheet formula prefixes at CSV export time
• protect CSV Converter, SMTP, and SMTP Batch exports by default
• preserve the original event data
• provide an explicit escape_csv_injection compatibility opt-out
• add regression tests and documentation

Security impact

Untrusted threat-feed values beginning with spreadsheet formula characters could be interpreted as formulas when an analyst opens an exported CSV file.

This change prefixes dangerous values with a single quote at the CSV export boundary. Stored IntelMQ event data remains unchanged.

Testing

• git diff --check
• python -m compileall -q intelmq
• targeted pytest suite: 14 passed, 13 subtests passed

Addresses the IntelMQ export paths discussed in #1652. External tools mentioned in that issue remain out of scope.