Librarian: busy-aware ping + per-query lost-result watchdog

Two refinements to the librarian health/delivery story, matching how it
actually behaves under load:

1. Busy-aware ping (case b - broken return path). A ping arriving while
   the worker is grinding a search no longer queues behind it (which made
   a healthy-but-busy librarian time out and look dead). The librarian
   tracks worker_busy and, when set, pongs back IMMEDIATELY without
   touching the queue. Being busy is fine - you can keep piling searches
   on. The ping still travels the librarian->bot return path, so it keeps
   catching the one thing it must: a disrupted/incompatible return path
   where queries vanish. Idle pings still go through the internal queue.

2. Per-query watchdog (case a - finished but result lost). The librarian
   now tracks every search uuid's lifecycle (queued -> processing ->
   gone) in active_queries, exposed via a new POST /query_status. After
   dispatching a search the bot records it in self.pending; watch_pending
   polls /query_status for each. While the librarian still knows the uuid
   the search is progressing - left alone. The moment a uuid VANISHES
   there while still pending on the bot, its result was computed but never
   delivered: after a grace window (to rule out an in-flight result) the
   bot posts a notice to the channel - but ONLY then. A normally delivered
   result is popped from self.pending by check_data_q and never flagged.

Hardening: the worker's search body is now wrapped in try/except/finally
so a crashing search can't kill the worker thread (which would freeze the
queue), and worker_busy / active_queries are always cleared. The grace
logic lives in a dependency-free librarian_watchdog.pending_verdict so it
is unit-testable without discord/pdf libs. /ping and /query_status are
plain (sync) views so they run without flask[async].

Tests: unit test_librarian_watchdog (verdict transitions); integration
test_librarian_query_lifecycle (query_status known/unknown + auth,
idle-ping-queues, busy-ping-pongs-directly). Suite: 28 integration + 48
unit green.

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

constants.py

@@ -93,6 +93,9 @@ SEND_QUERY = "/query"
 # Health-check round-trip: a pseudo-query that the librarian must pull off its
 # own internal queue and answer (same uuid) WITHOUT running a real search.
 LIBRARIAN_PING = "/ping"
+# Per-query watchdog probe: "do you still know this uuid?" (queued/processing).
+# A uuid that vanishes here without its result reaching the bot was lost.
+QUERY_STATUS = "/query_status"
 TIME_BETWEEN_CALLS = 100000
 LAST_SPONTANEOUS_CALL = datetime.now()