refactor(pep8): enforce full PEP8 compliance across backend Python codebase

- ruff.toml: select E/W/F/I/N rules, line-length=120, drop legacy ignores
- Auto-fix: sort 82 import blocks (isort), remove 29 unused imports,
  strip 6 trailing-whitespace blank lines in docstrings
- main.py: move setup_logging and settings imports to top (E402)
- errors.py: noqa N818 on DDD exception names (96 call sites, safe)
- intel_service.py: noqa N817 for universal ET alias
- atomic/elastic/sigma import services: move _MAX_UNCOMPRESSED_SIZE and
  _MAX_ENTRIES to module level (N806)
- compliance_import_service.py: move SAMPLE_CONTROLS / CIS_CONTROLS to
  module level; wrap long description strings (N806 + E501)
- snapshot_service.py: move STATUS_ORDER dict to module level (N806)
- sigma_import_service.py: remove dead dedup_key expression (F841)
- threat_actor_import_service.py: remove dead stix_to_actor expression (F841)
- data_source.py, seed_demo.py, campaign_scheduler_service.py,
  lolbas_import_service.py: wrap lines exceeding 120 chars (E501)
- d3fend_import_service.py: per-file E501 ignore (data file with long strings)

All 439 unit tests pass. ruff check app/ → All checks passed!

backend/app/services/report_engine.py

@@ -3,9 +3,9 @@
 Uses WeasyPrint for PDF generation and docxtpl for DOCX.
 """
 
+import logging
 import os
 import uuid
-import logging
 from datetime import datetime
 
 from jinja2 import Environment, FileSystemLoader
@@ -34,7 +34,7 @@ class ReportEngine:
 
     def generate_pdf(self, template_name: str, context: dict) -> str:
         """Render HTML and convert to PDF with WeasyPrint."""
-        from weasyprint import HTML, CSS
+        from weasyprint import CSS, HTML
 
         html_content = self.render_html(template_name, context)
         css_path = os.path.join(settings.REPORT_TEMPLATES_DIR, "styles", "report.css")