feat(reporting): Enhance CSV, extend JSON schema, and add Markdown export

Description:

Nettacker already supports HTML, CSV and JSON exports as part of its comprehensive reporting features. These formats are a major strength compared to other tools. However, there are opportunities to enhance and extend reporting to make results more reliable and more useful for modern workflows.

Motivation:

• CSV: While supported, there have been bug reports (e.g., API crashes) and inconsistencies. Stabilizing CSV export will improve reliability.

• JSON: Current JSON output is useful, but extending it into a SIEM‑friendly schema (with fields like severity, CVE, tags, timestamp) would allow direct ingestion into Splunk/ELK.

• Markdown: A new format, useful for sharing results in GitHub issues, documentation, or wikis.

Scope:

• Keep implementation modular in a new reporting module (e.g., nettacker/lib/report_export/).

• Avoid changes to core scanning logic.

• Add CLI option --report-format with choices: html, json, csv, markdown, siem-json.

• Default remains HTML/JSON for backward compatibility.

Deliverables:

• CSV stabilization — fix bugs and ensure consistent schema across CLI and API.

• Markdown export — generate tables for easy sharing.

• Extended JSON schema — add SIEM‑friendly fields.

• Documentation updates in docs/Usage.md.

• Unit tests for each format.

Example outputs for each format.

Non‑Goals:

No changes to core scanning logic.

No external dependencies beyond Python standard library.

No dashboard or visualization work (future scope).

Benefits:

• Builds on Nettacker’s existing strengths (CSV/JSON).

• Adds new formats (Markdown, SIEM‑JSON) for broader usability.

• Improves reliability and integration with modern security workflows.

Testing:

• Verify CSV export works for CLI and API across multiple modules/targets.

• Validate JSON schema against SIEM ingestion requirements.

• Confirm Markdown tables render correctly.

• Add unit tests for each format.

Start by locating the existing CLI and API report-export paths, then review the proposed nettacker/lib/report_export/ module and docs/Usage.md. Identify current format tests before defining the modular scope. Done means the requested formats, CLI option, documentation, examples, and unit tests are covered without changing scanning logic.