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/source_management.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net/http"
 	"strconv"
+	"strings"
 
 	"github.com/go-chi/chi"
 	"github.com/google/uuid"
@@ -903,7 +904,9 @@ func (h *Handlers) buildTargetConfig(
 		return h.buildHTTPTargetConfig(w, r, targetURL)
 	case database.TargetTypeSlack:
 		return h.buildSlackTargetConfig(w, r, targetURL)
-	case database.TargetTypeDatabase, database.TargetTypeLog:
+	case database.TargetTypeDatabase:
+		return h.buildDatabaseTargetConfig(w, r)
+	case database.TargetTypeLog:
 		return "", nil
 	default:
 		http.Error(
@@ -1013,6 +1016,47 @@ func (h *Handlers) buildSlackTargetConfig(
 	return string(configBytes), nil
 }
 
+// buildDatabaseTargetConfig builds config JSON for a database
+// (archive) target. The optional expiry form value is validated
+// here, at creation time, so an unparseable value is rejected
+// with a 400 instead of failing every subsequent delivery. An
+// empty expiry yields an empty config (the keep-forever
+// default).
+func (h *Handlers) buildDatabaseTargetConfig(
+	w http.ResponseWriter,
+	r *http.Request,
+) (string, error) {
+	expiry := strings.TrimSpace(r.FormValue("expiry"))
+	if expiry == "" {
+		return "", nil
+	}
+
+	err := delivery.ValidateArchiveExpiry(expiry)
+	if err != nil {
+		http.Error(
+			w,
+			"Invalid archive expiry: "+err.Error(),
+			http.StatusBadRequest,
+		)
+
+		return "", err
+	}
+
+	cfg := map[string]any{"expiry": expiry}
+
+	configBytes, err := json.Marshal(cfg)
+	if err != nil {
+		http.Error(
+			w, "Internal server error",
+			http.StatusInternalServerError,
+		)
+
+		return "", err
+	}
+
+	return string(configBytes), nil
+}
+
 // HandleEntrypointDelete handles deleting an entrypoint.
 func (h *Handlers) HandleEntrypointDelete() http.HandlerFunc {
 	return h.deleteChildResource(