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!

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

backend/app/domain/errors.py

@@ -46,7 +46,7 @@ class DuplicateEntityError(DomainError):
 # ── State machine ────────────────────────────────────────────────────
 
 
-class InvalidStateTransition(DomainError):
+class InvalidStateTransition(DomainError):  # noqa: N818 — DDD term, renaming would break 96 call sites
     """A state-machine transition is not allowed."""
 
     def __init__(
@@ -67,7 +67,7 @@ class InvalidStateTransition(DomainError):
 # ── Business rules ────────────────────────────────────────────────────
 
 
-class BusinessRuleViolation(DomainError):
+class BusinessRuleViolation(DomainError):  # noqa: N818 — DDD term, renaming would break 96 call sites
     """An operation violates a business invariant."""
 
     def __init__(self, message: str) -> None:
@@ -89,7 +89,7 @@ class InvalidOperationError(BusinessRuleViolation):
 # ── Authorization ────────────────────────────────────────────────────
 
 
-class PermissionViolation(DomainError):
+class PermissionViolation(DomainError):  # noqa: N818 — DDD term, renaming would break 96 call sites
     """The user lacks permissions for an action."""
 
     def __init__(self, message: str = "Insufficient permissions") -> None:

backend/app/domain/ports/repositories/test_repository.py

@@ -6,7 +6,7 @@ This is a domain contract — implementations live in infrastructure/.
 from __future__ import annotations
 
 import uuid
-from typing import Protocol, runtime_checkable
+from typing import Protocol
 
 from app.domain.enums import TestState
 

backend/app/domain/test_entity.py

@@ -34,7 +34,6 @@ from app.domain.errors import (
     InvalidStateTransition,
 )
 
-
 # ── Value objects ────────────────────────────────────────────────────