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/__init__.py

@@ -23,7 +23,7 @@ from .plugins import plugin_manager
 # 0.7.0: added the four ADR-010 frontend-contribution hooks (get_settings_cards,
 # get_asset_panels, get_map_overlays, get_asset_presentation), consumed by the
 # GET /api/pluginui/* endpoints. Four additive optional hooks, one minor bump.
-__contract_version__ = '0.7.0'
+__contract_version__ = '0.8.0'
 
 # Product release version (see ADR-007). The product version and the
 # plugin-contract version above are distinct series with independent