Use one recovery routine at start and on every sweep tick (closes #542)

Start-up recovery and the 60-second sweep were two parallel sets of
functions doing the same job. recoverDeliveries now does it, run once at
start and then on every tick from one goroutine. A retrying delivery is
scheduled after what is left of its backoff, now if nothing is left, so
backoffElapsed is gone. The run at start still takes every pending
delivery and opens a webhook database whose file is missing; a tick
still takes only pending deliveries untouched for 15 minutes, at most
500, and skips a missing file. A retrying delivery is taken before its
event is read. The README describes one routine.

Model: opus-5-5

internal/delivery/inflight.go

@@ -6,10 +6,10 @@ import "sync"
 //
 // A delivery is owned from the moment a task for it is handed to a
 // channel or to a retry timer until the engine has no further plan for
-// it in memory. Restart recovery and both arms of the periodic sweep
-// re-dispatch only deliveries the set does not hold, which is what
-// makes them exact rather than a guess about how long a row has sat at
-// pending.
+// it in memory. The recovery routine, at start and on every tick of the
+// sweep, re-dispatches only deliveries the set does not hold, which is
+// what makes it exact rather than a guess about how long a row has sat
+// at pending.
 //
 // This replaces reasoning from timestamps. A delivery's row says
 // pending from creation until its outcome is written, which covers
@@ -61,9 +61,9 @@ func (s *inflightSet) retain(deliveryID string) {
 
 // retainIdle takes the first reference on a delivery, and reports
 // whether it got it. It fails when the engine already owns the
-// delivery, which is what makes two claimants — restart recovery and
-// the sweep run concurrently, or two sweep arms — mutually exclusive
-// rather than merely atomic.
+// delivery, which is what makes two claimants — Notify and the
+// recovery routine, say — mutually exclusive rather than merely
+// atomic.
 func (s *inflightSet) retainIdle(deliveryID string) bool {
 	s.mu.Lock()
 	defer s.mu.Unlock()