feat(ssr): add delete selected to external ledger

Replicate the master CLJS "delete external ledger" feature on the SSR
external ledger page: an admin-only bulk delete that retracts the
selected journal entries, skipping any in a client's locked period and
capping at 1000 per request.

Return the result via modal-response (retargets the persistent
#modal-content shell) and target #modal-content from the button so the
request never relies on the outerHTML swap inherited from the data-grid
card, which previously replaced #modal-holder and broke the next click.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/cljc/auto_ap/routes/ledger.cljc

@@ -8,6 +8,7 @@
                      "/line-item" {:get ::new-line-item}}
 
              "/external-new" ::external-page
+             "/bulk-delete" ::bulk-delete
              "/external-import-new" {"" ::external-import-page
                                      "/parse" ::external-import-parse
                                      "/import" ::external-import-import}