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/snapshots.py

@@ -52,7 +52,7 @@ def list_snapshots(
     limit: int = Query(50, ge=1, le=200),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> list:
     """List coverage snapshots ordered by creation date (newest first)."""
     return list_snapshots_svc(db, offset=offset, limit=limit)
 
@@ -66,7 +66,7 @@ def create_snapshot_endpoint(
     payload: SnapshotCreate,
     db: Session = Depends(get_db),
     current_user: User = Depends(require_any_role("red_lead", "blue_lead", "admin")),
-):
+) -> dict:
     """Create a manual coverage snapshot with an optional name."""
     snapshot = create_snapshot(db, name=payload.name, user_id=current_user.id)
 
@@ -94,7 +94,7 @@ def coverage_evolution(
     months: int = Query(12, ge=1, le=36),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> list:
     """Return coverage snapshots for trend charts (last *months* months)."""
     return get_coverage_evolution(db, months=months)
 
@@ -109,7 +109,7 @@ def compare_snapshots_endpoint(
     b: str = Query(..., description="Snapshot B ID"),
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> dict:
     """Compare two snapshots showing improved, worsened, and unchanged techniques."""
     try:
         a_id = uuid.UUID(a)
@@ -129,7 +129,7 @@ def get_snapshot(
     snapshot_id: str,
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
-):
+) -> dict:
     """Get detailed snapshot information including per-technique states."""
     return get_snapshot_detail(db, snapshot_id)
 
@@ -143,7 +143,7 @@ def delete_snapshot_endpoint(
     snapshot_id: str,
     db: Session = Depends(get_db),
     current_user: User = Depends(require_role("admin")),
-):
+) -> dict:
     """Delete a snapshot (admin only)."""
     snapshot = get_snapshot_or_raise(db, snapshot_id)