Add photo management for models and employees; fix stale detail navigation

Model photos: upload/replace/delete on /api/models/<id>/image (admin),
stored under instance/modelimages/ with a public serve route; thumbnail
plus Upload/Replace/Remove controls in the Models settings modal; the
URL field remains as a manual alternative.

Employee photos, mode-aware: self-hosted directory employees get
upload/replace/delete (photo-<sso> under instance/employeephotos/,
employees plugin migration 0002); external directory mode passes the
HR-supplied picture URL through read-only (writes 409). One resolver
feeds both consumers - the shopfloor recognition/recert kiosk cards and
the employee detail hero - in either mode.

Navigation fix: router-view is keyed on route path, so following a
relationship link between two assets of the same type (machine ->
dualpath machine) reloads the page instead of showing stale content;
query-only URL changes still avoid a remount.

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

tests/test_plugin_migrations.py

@@ -49,6 +49,8 @@ EXPECTED_HEAD_REVISION['measuringtools'] = 'measuringtools0001baseline'
 # machines (renamed from equipment) keeps its original anchor id and adds the
 # rename revision on top, so its head is not the f-string default.
 EXPECTED_HEAD_REVISION['machines'] = 'machines0002rename'
+# employees adds the photofilename column on top of its cutover anchor.
+EXPECTED_HEAD_REVISION['employees'] = 'employees0002photo'
 
 # Plugins built after the cutover: their 0001 baseline really creates tables the
 # core chain never owned.