printedparts stage 11: low-stock email alerts on threshold crossing

Contract 0.12.0: send_email/send_alert join the plugin surface (the
mailer was core-only), PLUGIN-HOOKS and status docs updated, manifest
pins the new floor. The alert fires inside _ledger_write only when a
decrement CROSSES the item's threshold - one alert per depletion,
rearmed by restocking above - and is best-effort after the commit so
mail trouble can never fail a take. Recipients come from
printedparts_alert_email, falling back to the site alert_recipients.
on_enable re-seeds settings idempotently so existing installs pick up
new keys. Crossing/rearm semantics proven by test.

tests/test_plugins/test_printedparts_ledger.py

@@ -166,3 +166,37 @@ def test_kiosk_take_is_open_decrement_only(client, auth_headers, app, item,
                         PrintedItemTransaction.query.filter_by(
                             printeditemid=item).all())
         assert cached == ledgersum
+
+
+def test_lowstock_alert_fires_on_crossing_only(client, auth_headers, app, item,
+                                               directory_employee, monkeypatch):
+    """One alert when stock CROSSES the threshold downward; restocking above
+    rearms it; staying below does not re-fire."""
+    sent = []
+    import plugins.printedparts.api.routes as printedparts_routes
+    monkeypatch.setattr(
+        printedparts_routes, '_send_lowstock_alert',
+        lambda alerted_item: sent.append(alerted_item.itemcode))
+
+    def restock(quantity):
+        return client.post(f'/api/printedparts/items/{item}/restock',
+                           json={'quantity': quantity,
+                                 'badge': directory_employee},
+                           headers=auth_headers)
+
+    def take(quantity):
+        return client.post('/api/printedparts/kiosk/take',
+                           json={'itemcode': '3DP-9001',
+                                 'badge': directory_employee,
+                                 'quantity': quantity})
+
+    restock(10)                      # 10 on hand, threshold 5
+    assert take(3).status_code == 200   # 7: above threshold, no alert
+    assert sent == []
+    assert take(3).status_code == 200   # 4: CROSSES 5 -> one alert
+    assert sent == ['3DP-9001']
+    assert take(2).status_code == 200   # 2: still below, no re-fire
+    assert sent == ['3DP-9001']
+    restock(20)                          # 22: rearmed
+    assert take(18).status_code == 200  # 4: crosses again -> second alert
+    assert sent == ['3DP-9001', '3DP-9001']