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/delivery/target_database_archive.go

@@ -37,6 +37,13 @@ var (
 	errArchiveNoDataDir = errors.New(
 		"database target has no data directory",
 	)
+
+	// errArchiveExpiryNotPositive is returned when a
+	// user-supplied archive expiry parses as a duration but is
+	// zero or negative; "never" is the way to disable pruning.
+	errArchiveExpiryNotPositive = errors.New(
+		"expiry must be a positive duration or \"never\"",
+	)
 )
 
 // databaseTargetConfig is the optional per-target JSON config
@@ -105,6 +112,35 @@ func parseArchiveExpiry(
 	return dur, nil
 }
 
+// ValidateArchiveExpiry checks a user-supplied archive expiry
+// for a database target at configuration time. Valid values are
+// empty, "never" (both meaning keep forever), or a positive Go
+// duration such as "720h". Anything else is an error, so a bad
+// expiry is rejected when the target is created rather than
+// failing every subsequent delivery.
+func ValidateArchiveExpiry(expiry string) error {
+	if expiry == "" || expiry == archiveExpiryNever {
+		return nil
+	}
+
+	dur, err := time.ParseDuration(expiry)
+	if err != nil {
+		return fmt.Errorf(
+			"expiry must be %q or a Go duration "+
+				"such as \"720h\": %w",
+			archiveExpiryNever, err,
+		)
+	}
+
+	if dur <= 0 {
+		return fmt.Errorf(
+			"%w: %q", errArchiveExpiryNotPositive, expiry,
+		)
+	}
+
+	return nil
+}
+
 // archiveWriter owns one per-webhook archive SQLite file. It
 // serialises writes, and after each write closes and reopens
 // the file (debounced to at most once per debounce window) so