Add email sending (service + 3 flows) and a general asset label generator

Email: a stdlib SMTP mailer (settings-first config, graceful no-op when
unconfigured), a test-email endpoint wired to the Email settings page,
forced first-login password change (users.mustchangepassword, migration
7d23, /change-password flow), new-user welcome mail, and on-demand
report/alert delivery (POST /api/reports/email + Email Report buttons)
with an external-cron-with-a-scoped-PAT path documented for automation.
All tests patch smtplib - no network.

Labels: a shared /print/asset-label/<type>/<id> view any asset detail
page opens - card or plain style, QR or barcode, configurable encoding.
Per-type qr_target_* templates plus label_default_style/codetype/encodes
settings on the Printing page. Measuring-tool labels default to encoding
their inspection-operation code (derived from the location name, e.g.
0615), so every tool in an area shares the area code - verified by
decoding the rendered QR. Machine labels default to the machine number;
blank-serial handled gracefully.

808 tests pass; both features verified live.

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

frontend/src/views/printers/PrinterDetail.vue

@@ -6,6 +6,9 @@
         <router-link :to="`/print/printer-qr/${$route.params.id}`" class="btn btn-secondary" target="_blank">
           Print QR
         </router-link>
+        <router-link :to="`/print/asset-label/printer/${$route.params.id}`" class="btn btn-secondary" target="_blank">
+          Print Label
+        </router-link>
         <router-link :to="`/printers/${$route.params.id}/edit`" class="btn btn-primary">Edit</router-link>
         <router-link to="/printers" class="btn btn-secondary">Back to List</router-link>
       </div>