Align docs with archive semantics; fail loud on non-positive expiry (#43)

- README: rewrite the database-target documentation (target-types
  bullet and the per-webhook databases section) to describe the
  shipped archiving semantics -- separate archive-{webhookID}.db,
  debounced close/reopen for offline archiving, auto-recreate,
  creation-validated optional expiry with prune-on-open, and
  fail-loud delivery on archive write errors -- replacing the
  stale always-successful stub description.
- parseArchiveExpiry now returns an error for set-but-non-positive
  durations ("0s", "-5h") instead of silently defaulting to
  keep-forever, matching ValidateArchiveExpiry at creation time;
  the delivery then fails loudly like any other archive error.
  TestParseArchiveExpiry extended with zero and negative cases.
- databaseTarget type comment: "fire-and-forget" -> "no-retry",
  matching the fail-loud behaviour.

internal/delivery/target_database_archive.go

@@ -77,7 +77,10 @@ type archivedEvent struct {
 // parseArchiveExpiry reads the optional expiry from a database
 // target's config JSON. An empty config, an empty expiry, or
 // the literal "never" all mean keep forever, returned as a zero
-// duration. Any other value is parsed as a Go duration.
+// duration. Any other value must parse as a positive Go
+// duration; a set-but-invalid value (unparseable, zero, or
+// negative) is an error rather than a silent default, matching
+// ValidateArchiveExpiry at target creation.
 func parseArchiveExpiry(
 	configJSON string,
 ) (time.Duration, error) {
@@ -106,7 +109,9 @@ func parseArchiveExpiry(
 	}
 
 	if dur <= 0 {
-		return 0, nil
+		return 0, fmt.Errorf(
+			"%w: %q", errArchiveExpiryNotPositive, cfg.Expiry,
+		)
 	}
 
 	return dur, nil