printedparts stage 13: pick alert recipients from shopdb users

Contract 0.13.0 puts the User model on the plugin surface. The
settings page gains a checkbox picker over the user list; selected
users receive low-stock alerts at their account email, merged and
deduped with the free-text address list, inactive accounts skipped,
site alert_recipients still the fallback when both are empty.

docs/PLUGIN-LAB-PRINTEDPARTS.md

@@ -320,6 +320,21 @@ it. The page is ordinary:
 3. `get_settings_cards` on the plugin pointing at `/settings/printedparts` -
    the card appears in the rail's catalog while the plugin is enabled.
 
+## Stage 13 (extension) - alert recipients picked from shopdb users
+
+Free-text emails rot; user accounts do not. Another contract addition:
+`User` joins the surface (0.13.0 - export, PLUGIN-HOOKS, version bump, the
+docs-drift guard again).
+1. Setting `printedparts_alert_userids` (comma-separated user ids), seeded
+   beside the others.
+2. `_alert_recipients()`: resolve each selected id to an ACTIVE user's
+   account email, merge with the free-text list, dedupe order-preserving;
+   empty result still falls back to the site alert_recipients.
+3. Settings page: checkbox picker over `usersApi.list()` (the page is
+   admin-only, matching the endpoint), saving joined ids.
+4. Test: active user's email + free-text merge deduped, inactive user
+   skipped (`test_alert_recipients_merge_users_and_freetext`).
+
 ---
 
 ## Where each pattern lives (cheat sheet)