fix(i18n): every counted string is a plural pair, so one reads singular

"1 seasons", "Synced 1 subscriptions", "1 video(s) were skipped": 21 English
strings carried {{count}} in one plain key and read plural at one. Each is a
_one/_other pair now (Hungarian keeps one form for both), and so is every other
counted string, so the next wording change is a plural too. A contract test
holds the rule for every locale file.

frontend/src/i18n/locales/en/scheduler.json

@@ -14,7 +14,8 @@
   "jobsTitle": "Jobs",
   "queueTitle": "Work queued",
   "quotaTitle": "Shared API quota",
-  "everyMin": "every {{count}} min",
+  "everyMin_one": "every {{count}} min",
+  "everyMin_other": "every {{count}} min",
   "runningNow": "Running now…",
   "lastRun": "Last run {{time}}",
   "neverRun": "Hasn't run yet this session",
@@ -115,7 +116,8 @@
   "purgeDiscovery": "Purge discovery",
   "purgeDiscoveryHint": "Delete all un-kept discovery content now (search results nobody watched/saved, and explored-but-unsubscribed channels) — ignoring the grace period.",
   "purgeConfirmTitle": "Purge discovery content?",
-  "purgedDiscovery": "Deleted {{count}} discovery items",
+  "purgedDiscovery_one": "Deleted {{count}} discovery item",
+  "purgedDiscovery_other": "Deleted {{count}} discovery items",
   "result": {
     "new": "new videos",
     "failed": "failed",