Mask the webhook credential in delivery errors and logs (closes #118)

Go embeds the request URL in *url.Error, so any transport failure — DNS,
TLS, refused, timeout, SSRF dial block — persisted the full Slack webhook
URL into the per-webhook SQLite database via DeliveryResult.Error. That
field is tagged json:"error,omitempty", so a future REST API would have
served it.

maskURLError rebuilds the error preserving Op and the wrapped cause, so DNS
vs TLS vs timeout still read differently and errors.Is/As and Timeout()
keep working; only path, query and userinfo are dropped. Applied where the
errors are born, which covers both the Slack and HTTP targets. url.Parse
embeds the URL too, so ValidateTargetURL's parse branch gets the same
treatment.

The SSRF rejection log now logs the masked URL, and source_logs.html
receives view types rather than raw rows, so no config blob is reachable
from that template.

MaskURL is now the single masker for the whole tree.

internal/handlers/source_detail_test.go

@@ -26,14 +26,14 @@ const (
 )
 
 // seedConfiguredTarget inserts a target with a stored config
-// blob.
+// blob and returns it.
 func seedConfiguredTarget(
 	t *testing.T,
 	db *database.Database,
 	webhookID string,
 	targetType database.TargetType,
 	config string,
-) {
+) *database.Target {
 	t.Helper()
 
 	tgt := &database.Target{
@@ -48,6 +48,8 @@ func seedConfiguredTarget(
 		t,
 		db.DB().Omit(clause.Associations).Create(tgt).Error,
 	)
+
+	return tgt
 }
 
 // renderSourceDetailPage runs the real source detail handler