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/ruff.toml

@@ -1,13 +1,21 @@
+# PEP8 line length: 120 chars — the codebase uses longer identifiers and SQLAlchemy chaining
+line-length = 120
+
 [lint]
-# Ignore rules that have widespread pre-existing violations.
-# These can be cleaned up incrementally in follow-up PRs.
+# PEP8 compliance rules enforced:
+# E/W — pycodestyle (core PEP8 style and warnings)
+# F   — pyflakes (unused imports, undefined names)
+# I   — isort (import ordering per PEP8 convention)
+# N   — pep8-naming (class/function/variable naming conventions)
+select = ["E", "W", "F", "I", "N"]
+
 ignore = [
-    "E402",   # module-level import not at top of file (app.main, some services)
-    "E712",   # == True comparisons (required by SQLAlchemy filter syntax)
-    "F401",   # unused imports (widespread; clean up incrementally)
-    "F841",   # unused local variables (a few occurrences)
+    # SQLAlchemy filter syntax requires `== True` / `== False` comparisons
+    "E712",
 ]
 
 [lint.per-file-ignores]
-# Test files may use broad exception catching and unusual import patterns
-"tests/**" = ["E", "F"]
+# Tests use broad exception catching and unusual import patterns
+"tests/**" = ["E", "F", "N"]
+# Data file: D3FEND technique descriptions contain URLs and long strings that cannot be meaningfully wrapped
+"app/services/d3fend_import_service.py" = ["E501"]