fix(search): every related and live-search request takes a slot, and a failed one gives it back

A related list's cursor cost nothing, and a cursor is a free string: a replayed one read and
enriched without end. Every request now takes a slot of the user's daily cap — taken under a lock
before the work (`quota.claim_action`), so requests sent together cannot all pass the last free
one, and given back when the request fails outright (`quota.refund_action`). The live search's
scrape source takes its slots the same way.

A limit of 0 turns the feature off: a distinct 409 `related_disabled` / `search_disabled`, and
`/api/me` says `related_enabled` / `search_enabled` so the UI can leave it out.

A videos.list that fails no longer reads as an empty page: `ingest_candidates` raises, and the
search and the related list answer the refusal (the shared budget spent says "quota") instead of
"no results". The two routes' page loops are one (`_gather`).

`clear` deletes only what the caller found: a related list's rows carry no find, and any user
could name them.

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

@@ -188,7 +188,9 @@
     "quota_spent": "Today's shared YouTube budget is spent. Try again tomorrow.",
     "recipient_required": "A recipient email is required",
     "registration_closed": "Registration is currently closed",
+    "related_disabled": "Related videos are turned off on this instance.",
     "related_daily_limit": "You've opened today's limit of related lists ({{limit}}). Try again tomorrow.",
+    "search_disabled": "Live YouTube search is turned off on this instance.",
     "search_daily_limit": "You've reached today's YouTube search limit ({{limit}}). Try again tomorrow.",
     "search_term_required": "Enter something to search for",
     "session_gone": "The playback session ended",