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>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.128.18"
+version = "0.128.19"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.128.18"
+version = "0.128.19"
 edition = "2021"
 default-run = "mangalord"
 

backend/migrations/0039_crawler_jobs_running_lease_idx.sql

@@ -0,0 +1,18 @@
+-- Index the crashed-worker-recovery arm of the job lease predicate.
+--
+-- lease/lease_kinds match:
+--   WHERE (state = 'pending' OR (state = 'running' AND leased_until < now()))
+-- crawler_jobs_ready_idx (0016) is `ON (scheduled_at) WHERE state = 'pending'`,
+-- so it covers only the pending arm. The `state = 'running' AND leased_until`
+-- arm had no usable index, so Postgres could not BitmapOr the two arms and
+-- degraded to a sequential scan of the ENTIRE crawler_jobs table — including all
+-- done/dead rows not yet reaped — on every lease poll, by every worker,
+-- continuously (audit H2, the headline performance finding).
+--
+-- A partial index on leased_until over just the running rows makes the recovery
+-- arm index-backed. `state = 'running'` is an immutable predicate (now() stays
+-- in the query, not the index). The running set is tiny (in-flight jobs only),
+-- so the index is cheap to maintain.
+CREATE INDEX crawler_jobs_running_lease_idx
+    ON crawler_jobs (leased_until)
+    WHERE state = 'running';

frontend/package.json

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