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_generation_service.py

@@ -98,7 +98,7 @@ def generate_coverage_report(
     output_format: str = "pdf",
 ) -> str:
     """Generate an organization-wide MITRE ATT&CK coverage report."""
-    from sqlalchemy import func, case
+    from sqlalchemy import case, func
 
     org_score = _safe_org_score(db)
 
@@ -234,7 +234,8 @@ def generate_quarterly_summary(
     output_format: str = "pdf",
 ) -> str:
     """Quarterly summary — reuses executive metrics plus snapshot trend rows."""
-    from sqlalchemy import case as sql_case, func
+    from sqlalchemy import case as sql_case
+    from sqlalchemy import func
 
     org_score = _safe_org_score(db)
     quarter_ago = datetime.utcnow() - timedelta(days=90)