Librarian: graceful shutdown with resumable search state

A restart of the librarian used to throw away an in-flight search (and any
searches still queued). Now search state survives a restart:

* Resumable DB scan (search_bot): each producer records a tell()-cookie
  watermark per chunk file as it goes (safe because search_for_doi drains
  the work queue before returning), and can seek back to it. search_for_doi
  now takes stop_event + resume and returns (result_list, positions,
  interrupted).

* Persisted requests: /query writes the accepted request to a disk queue
  before enqueuing; replay_requests re-enqueues unfinished ones on startup.
  So even a search still waiting in the queue survives a restart.

* Checkpoints: when a graceful shutdown interrupts a scan, the librarian
  writes {dois, found-so-far, per-file offsets}. On restart answer_query
  loads it, skips the (already done) Crossref+refine, and continues the
  scan from the saved offsets with the found DOIs pre-marked - no line is
  read twice and none is missed. A finished or crashed search forgets its
  request+checkpoint (no poison-pill replay).

* Graceful shutdown: SIGTERM/SIGINT set a shutdown event; the running scan
  checkpoints and the worker stops. The main thread then exits within a
  BOUNDED window (CONJURER_LIBRARIAN_GRACEFUL_TIMEOUT, default 45s) so the
  pod can never become an un-killable zombie. Needs terminationGracePeriod
  >= that in the deploy (separate PR).

Tests: search_bot resume correctness (seek past scanned, don't miss/re-scan;
stop_event -> interrupted) and librarian state mechanics (request replay,
forget, checkpoint round-trip, poison-pill drop). Suite: 58 unit + 49
integration green.

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

durable_queue.py

@@ -52,6 +52,14 @@ class DiskQueue:
     def contains(self, key) -> bool:
         return os.path.exists(self._path(key))
 
+    def get(self, key):
+        """Return the payload stored for ``key``, or None if absent/unreadable."""
+        try:
+            with open(self._path(key), encoding="utf-8") as handle:
+                return json.load(handle)["payload"]
+        except (OSError, ValueError, KeyError, TypeError):
+            return None
+
     def remove(self, key) -> None:
         try:
             os.remove(self._path(key))