chore: code-review cleanups (NITs + filename defense-in-depth test)

- NIT-1: remove dead _technique_names() and _technique_ids() helpers (no callers)
- NIT-2: rename engagement → _engagement in render_engagement_csv signature
- NIT-4: remove duplicate inline User import in test_export_csv_escapes_special_characters
- NIT-5: add comment on _CSV_FORMULA_TRIGGERS explaining \t and \r inclusion
- REUSE: replace custom _html_escape with stdlib html.escape (quote=True default)
- Remove now-unnecessary type: ignore comments on weasyprint (stubs resolve cleanly)
- Add test_export_filename_never_contains_quote_or_crlf defense-in-depth test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/export.py

@@ -6,6 +6,7 @@ import io
 import re
 import unicodedata
 from datetime import date
+from html import escape as _html_escape
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
@@ -21,14 +22,6 @@ def _export_filename(engagement: Engagement, ext: str) -> str:
     return f"engagement-{engagement.id}-{slug}-{today}.{ext}"
 
 
-def _technique_names(techniques: list[dict]) -> str:
-    return " | ".join(t.get("name", t.get("id", "")) for t in (techniques or []))
-
-
-def _technique_ids(techniques: list[dict]) -> str:
-    return " | ".join(t.get("id", "") for t in (techniques or []))
-
-
 def _tactic_names(tactic_ids: list[str]) -> str:
     from backend.app.serializers import _enrich_tactics
 
@@ -154,6 +147,8 @@ _CSV_HEADERS = [
     "updated_at",
 ]
 
+# \t and \r included: Excel auto-trims leading whitespace, so a tab/CR prefix still
+# reaches the formula parser in some sheet versions.
 _CSV_FORMULA_TRIGGERS = ("=", "+", "-", "@", "\t", "\r")
 
 
@@ -172,7 +167,7 @@ def _csv_safe(value: object) -> object:
 
 
 def render_engagement_csv(
-    engagement: Engagement, simulations: list[Simulation]
+    _engagement: Engagement, simulations: list[Simulation]
 ) -> str:
     buf = io.StringIO()
     writer = csv.writer(buf)
@@ -223,15 +218,6 @@ pre { background: #f5f5f5; padding: 8px; font-size: 11px; white-space: pre-wrap;
 """
 
 
-def _html_escape(text: str) -> str:
-    return (
-        text.replace("&", "&")
-        .replace("<", "&lt;")
-        .replace(">", "&gt;")
-        .replace('"', "&quot;")
-    )
-
-
 def _render_engagement_html(
     engagement: Engagement, simulations: list[Simulation]
 ) -> str:
@@ -312,7 +298,7 @@ def _render_engagement_html(
 def render_engagement_pdf(
     engagement: Engagement, simulations: list[Simulation]
 ) -> bytes:
-    from weasyprint import HTML  # type: ignore[import-untyped]
+    from weasyprint import HTML
 
     html = _render_engagement_html(engagement, simulations)
-    return HTML(string=html).write_pdf()  # type: ignore[no-any-return]
+    return HTML(string=html).write_pdf()

backend/tests/test_export_engagement.py

@@ -136,8 +136,6 @@ def test_export_csv_escapes_special_characters(
     eng = _make_engagement(client, admin_token)
 
     with app.app_context():
-        from backend.app.models import User
-
         admin = User.query.filter_by(username="admin1").first()
         sim = Simulation(
             engagement_id=eng["id"],

backend/tests/test_export_render.py

@@ -247,3 +247,16 @@ def test_render_engagement_csv_does_not_alter_safe_strings(app) -> None:
         cells = _parse_csv_data_row(result)
         assert cells[1] == "Mimikatz LSASS Dump", "safe name must not be modified"
         assert cells[6] == "whoami /all", "safe commands must not be modified"
+
+
+def test_export_filename_never_contains_quote_or_crlf() -> None:
+    """Defense-in-depth: even with malicious engagement names, the filename
+    used in Content-Disposition must never contain header-injection chars."""
+    from types import SimpleNamespace
+
+    from backend.app.services.export import _export_filename
+    evil = SimpleNamespace(id=42, name='evil"name\r\nX-Injected: yes')
+    fname = _export_filename(evil, "md")
+    assert '"' not in fname
+    assert '\r' not in fname
+    assert '\n' not in fname