feat(imports): add nmap XML importer

[rachit367]

What

Adds an nmap XML importer so an existing scan can be imported into a sketch, instead of re-scanning a target or adding ports by hand.

Closes #107

Why

From the issue: during assessments people often run nmap manually with specific flags, and naabu can also export in nmap XML. Letting Flowsint ingest that XML enriches a sketch with the hosts/ports already discovered. This reuses the existing two-phase import pipeline (analyze → execute) — no new infrastructure.

How it plugs in

The import pipeline dispatches parsers by file extension (flowsint_core/imports/file_parser.py) and each parser is a function returning a FileParseResult of typed entities + edges, which the pipeline persists. The new parser follows that exact contract (same shape as the existing JSON parser).

• flowsint_core/imports/nmap/parse_nmap.py (new) — parse_nmap_xml() turns an nmap report into:
  • Ip entities (one per up host; down hosts skipped; IPv4 preferred, IPv6 supported, MAC ignored)
  • Port entities (number, protocol, state, service, and a banner composed from service product/version/extrainfo)
  • IP -[HAS_PORT]-> Port edges (the same relationship label the ip_to_ports enricher uses)
• Wiring: .xml added to ALLOWED_EXTENSIONS (core dispatcher) + the analyze route's filename validation (flowsint-api/.../sketches.py) + the import sheet's accepted extensions (flowsint-app/.../import-sheet.tsx). Ip and Port already exist, so no type changes were needed.

Security

Import files are untrusted, so the XML is parsed with defusedxml (new flowsint-core dependency) to prevent XXE / billion-laughs attacks. Element type hints still come from the stdlib (identical types, not a parsing path).

Testing

uv run --package flowsint-core pytest flowsint-core/tests/import/ -q
56 passed   # 10 new nmap tests + existing import suite, no regressions

New tests (pure-parse, no DB): IP/port extraction, port fields + banner composition, HAS_PORT edges, down-host skipping, IPv6, invalid-port skipping, empty/invalid XML handling, and dispatch through parse_import_file.

Scope note: this maps the core of an nmap scan (hosts + ports + service info). NSE script output (which could map to the existing Script type) is intentionally left out to keep the PR focused; happy to add it as a follow-up if wanted.