Librarian: survive transient Crossref failures instead of losing the search

Field report: one httpx ReadTimeout inside habanero surfaced as
'Search <uuid> crashed', and the worker's crash handler then FORGOT the
request - so an expensive search vanished and the user was told it was
eaten, because a public API blinked once.

Two defences:
* Every habanero call goes through _crossref_call, which retries with
  linear backoff (CONJURER_CROSSREF_ATTEMPTS, default 4; backoff
  CONJURER_CROSSREF_BACKOFF, 5s). habanero wraps httpx errors in a plain
  RuntimeError so we can't filter narrowly - retries are simply bounded
  and the last error is re-raised. They now also run via asyncio.to_thread,
  so a slow Crossref no longer blocks the worker's event loop.
* A crashed search is no longer dropped on the first failure: the attempt
  count is persisted with the request and the search is requeued (keeping
  any checkpoint, so a crashed DB scan resumes rather than restarts) until
  CONJURER_SEARCH_MAX_ATTEMPTS (default 3). It stays 'queued' for the
  bot's watchdog while retrying, and only after the cap is it forgotten.

Also: scrape_bot's 'Got blocked' is routine sci-hub behaviour (it backs off
an hour and carries on) - log it as WARNING, not ERROR, so it stops looking
like a fault when scanning for real problems.

Tests: retry-then-succeed, bounded re-raise, no retry on success, the
persisted attempt counter, and forget-on-give-up. Suite: 58 unit + 70
integration green.

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

conjurer_librarian/scrape_bot.py

@@ -106,8 +106,10 @@ def check_if_exists_brute_force(logger):
         ):
             pass
         if blocked:
-            logger.info(item)
-            logger.error("Got blocked. Fuck.")
+            # Expected, routine sci-hub behaviour (we back off an hour and carry
+            # on) - WARNING, not ERROR, so it stops masquerading as a fault when
+            # you're scanning the log for real problems.
+            logger.warning("Got blocked. Fuck. Backing off an hour: %s", item[0])
             time.sleep(60 * 60)
         # trunk-ignore(bandit/B311)
         rand = random.randint(1, 60)