Fail deliveries on archive errors; validate expiry at creation (#43)

Two review findings on the database archiving target:

- An archive error now records the attempt as failed with the
  error string and marks the delivery failed, instead of logging
  the error and reporting success. A target that could not do its
  one job must not claim it did.
- The archive expiry is now actually configurable: the add-target
  form gains an expiry field for database targets, and the value
  is validated at creation time via the new
  delivery.ValidateArchiveExpiry (empty, "never", or a positive
  Go duration), rejecting bad values with a 400 at the only place
  a human can fix them, mirroring how Slack target URLs are
  validated at creation.

Test updates: a forced archive failure asserts a failed delivery
with a recorded error and no archive file; config builder tests
cover empty/never/duration and rejection paths; the two engine
tests that exercise the database target now build engines with a
real webhook DB manager since archiving is no longer a no-op; the
reopen-debounce test uses a wider window so parallel test load
cannot make two rapid writes straddle it.

internal/handlers/export_test.go

@@ -22,3 +22,13 @@ func (s *Handlers) BuildSlackTargetConfigForTest(
 ) (string, error) {
 	return s.buildSlackTargetConfig(w, r, targetURL)
 }
+
+// BuildDatabaseTargetConfigForTest exposes
+// buildDatabaseTargetConfig for use in the handlers_test
+// package.
+func (s *Handlers) BuildDatabaseTargetConfigForTest(
+	w http.ResponseWriter,
+	r *http.Request,
+) (string, error) {
+	return s.buildDatabaseTargetConfig(w, r)
+}