Add the API import surface for legacy migrations (contract 0.8.0)

Goal: an LLM or script can migrate an entire legacy database using only
the HTTP API - original history preserved, safely re-runnable.

- X-Import-Mode header (admin only): create/update endpoints across 15
  timestamped entity types accept original createddate/modifieddate;
  helper exposed via shopdb.api (contract 0.7.0 -> 0.8.0).
- Exact-match natural-key lookup filters on 13 list endpoints for the
  lookup-then-upsert recipe.
- Selfhosted USB checkout/checkin accept backdated event times in
  import mode.
- docs/IMPORT-API.md: operator manual grounded in the real legacy
  schema - order of operations, full table-by-table mapping including
  the machines fan-out, idempotent Python importer with dry-run, parity
  checks, and decided dispositions for unmigrated tables (DNC config
  stays live-fed via the collector; supportteams/appowners map to the
  upcoming supportteams model).

635 tests pass; naming green; frontend untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

shopdb/api/__init__.py

@@ -58,6 +58,13 @@ from shopdb.utils.pagination import get_pagination_params, paginate_query
 # Authorization decorators for gating plugin write routes
 from shopdb.utils.authz import require_permission, require_role
 
+# Import-mode helpers: preserve legacy timestamps during a bulk data import
+from shopdb.utils.import_mode import (
+    apply_import_timestamps,
+    import_mode_active,
+    parse_import_datetime,
+)
+
 # Legacy employee directory lookup (read-only) used by notifications
 from shopdb.utils.employee_db import employee_connection
 
@@ -239,6 +246,10 @@ __all__ = [
     # Authorization decorators
     'require_permission',
     'require_role',
+    # Import-mode helpers
+    'apply_import_timestamps',
+    'import_mode_active',
+    'parse_import_datetime',
     # Legacy employee directory
     'employee_connection',
     # CMMC USB check-in/out database