fix: index the running/leased_until arm of the job-lease query

The lease predicate's `state='running' AND leased_until < now()` arm had no
index, so every lease poll seq-scanned the whole crawler_jobs table (audit H2).
migration 0039 adds a partial index on (leased_until) WHERE state='running'.
Behavior-neutral; 29 lease tests green.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.18",
+  "version": "0.128.19",
   "private": true,
   "type": "module",
   "scripts": {