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/routers/snapshots.py

@@ -17,18 +17,19 @@ from app.dependencies.auth import get_current_user, require_any_role, require_ro
 from app.domain.errors import BusinessRuleViolation
 from app.domain.unit_of_work import UnitOfWork
 from app.models.user import User
-from app.services.snapshot_service import (
-    create_snapshot,
-    compare_snapshots,
-    cleanup_old_snapshots,
-    get_coverage_evolution,
-    serialize_snapshot_summary,
-    list_snapshots as list_snapshots_svc,
-    get_snapshot_or_raise,
-    get_snapshot_detail,
-    delete_snapshot,
-)
 from app.services.audit_service import log_action
+from app.services.snapshot_service import (
+    compare_snapshots,
+    create_snapshot,
+    delete_snapshot,
+    get_coverage_evolution,
+    get_snapshot_detail,
+    get_snapshot_or_raise,
+    serialize_snapshot_summary,
+)
+from app.services.snapshot_service import (
+    list_snapshots as list_snapshots_svc,
+)
 
 logger = logging.getLogger(__name__)