Try every event database file on delete and say which was left (closes #275)

DeleteDB stopped at the first file it could not remove, and the
webhook delete handler logged one message for every failure. Once
the database file itself is gone, its events are lost, and telling
the operator only that a file was left behind suggests otherwise.

DeleteDB now tries the database file and both sidecars, and its
error wraps ErrEventDBNotRemoved when the database file is still on
disk, or ErrSidecarNotRemoved when only a -wal or -shm sidecar is
left. The handler logs a different message for each.

Model: opus-5-5

internal/handlers/export_test.go

@@ -36,6 +36,15 @@ const MaxRenderedAttemptsForTest = maxRenderedAttempts
 // the handlers enforce rather than a number copied beside it.
 const MaxTargetRetriesForTest = maxTargetRetries
 
+// EventDBLeftMsgForTest and SidecarLeftMsgForTest expose the two
+// messages the webhook delete handler logs when a file of the event
+// database is left on disk, so a test checking that one is absent
+// checks for the handler's own wording.
+const (
+	EventDBLeftMsgForTest = eventDBLeftMsg
+	SidecarLeftMsgForTest = sidecarLeftMsg
+)
+
 // PageOrFirstForTest exposes pageOrFirst for use in the handlers_test
 // package.
 func PageOrFirstForTest(s string) int {