refactor(types): add comprehensive type annotations across backend Python codebase

Enable ANN rules in ruff.toml (flake8-annotations) and resolve all 221 violations:

ANN201/ANN202 — return types on 168 public/private functions:
- All 28 FastAPI routers: endpoints annotated with dict/list/specific schema/
  StreamingResponse/FileResponse/JSONResponse as appropriate
- main.py: lifespan→AsyncGenerator[None,None], exception handlers→JSONResponse
- database.py: get_db→Generator[Session,None,None], proxy methods→correct types
- middleware/request_context.py: dispatch→Response with Callable call_next type

ANN001/ANN002/ANN003 — 32 missing argument types:
- seed_demo.py: all db parameters typed as Session
- domain/unit_of_work.py: __aexit__ exc_type/exc_val/exc_tb typed with TracebackType
- services: audit_service user_id→UUID|None, heatmap_service query/model/builder,
  notification_service test→Test, tempo_service test→Test/user→User,
  test_workflow_service test_id→UUID, campaign_crud **fields→object,
  test_crud **fields→object (4 sites)

ANN401 — 16 Any usages resolved:
- Domain entities (campaign/technique/threat_actor/test_entity): replaced Any with
  actual ORM types via TYPE_CHECKING guards to avoid circular imports
- detection_rule_service: test_id/detection_rule_id/evaluator_id→UUID
- score_cache: kept Any with # noqa: ANN401 (genuinely generic cache)
- jira_service/tempo_service: kept Any with # noqa: ANN401 (lazy optional deps)
- d3fend_import_service: _to_str(v: Any) kept with # noqa: ANN401

ANN204/ANN205/ANN206 — special/static/class methods:
- database.py proxy __call__/__getattr__: *args: object/**kwargs: object
- schemas/test.py model_validate: obj→object, **kwargs→object
- sa_technique_repository._int_type→type

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

backend/app/routers/reports.py

@@ -38,7 +38,7 @@ def coverage_summary(
     platform: Optional[str] = Query(None, description="Filter by platform (in techniques)"),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> dict:
     """Full coverage report as JSON — technique-by-technique with test counts."""
     return build_coverage_summary(db, tactic=tactic, platform=platform)
 
@@ -49,7 +49,7 @@ def coverage_csv(
     platform: Optional[str] = Query(None),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> StreamingResponse:
     """Export coverage as a downloadable CSV."""
     rows = build_coverage_csv_rows(db, tactic=tactic, platform=platform)
 
@@ -74,7 +74,7 @@ def test_results(
     date_to: Optional[str] = Query(None, description="ISO date string YYYY-MM-DD"),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> dict:
     """Report of test results with optional filters."""
     return build_test_results_report(db, state=state, date_from=date_from, date_to=date_to)
 
@@ -84,6 +84,6 @@ def remediation_status(
     status: Optional[str] = Query(None, description="Filter by remediation status"),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> dict:
     """Report of remediation status across all tests."""
     return build_remediation_status_report(db, status=status)