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/delivery/target_config_view.go

@@ -3,7 +3,6 @@ package delivery
 import (
 	"encoding/json"
 	"fmt"
-	"net/url"
 	"strconv"
 
 	"sneak.berlin/go/webhooker/internal/database"
@@ -17,9 +16,6 @@ import (
 // browser history, screenshots and screen shares.
 const configUnavailable = "(unavailable)"
 
-// urlPathElision stands in for a URL's elided path.
-const urlPathElision = "/..."
-
 // ConfigField is one labelled, display-safe value derived
 // from a target's stored configuration.
 type ConfigField struct {
@@ -202,23 +198,5 @@ func databaseConfigFields(configJSON string) []ConfigField {
 // parse into a scheme and host yields the neutral
 // placeholder, never the raw string.
 func (c *SlackTargetConfig) MaskedWebhookURL() string {
-	return maskURL(c.WebhookURL)
-}
-
-// maskURL renders a URL as scheme plus host with everything
-// that can carry a secret removed.
-func maskURL(raw string) string {
-	parsed, err := url.Parse(raw)
-	if err != nil || parsed.Scheme == "" ||
-		parsed.Host == "" {
-		return configUnavailable
-	}
-
-	masked := parsed.Scheme + "://" + parsed.Host
-
-	if parsed.Path != "" && parsed.Path != "/" {
-		masked += urlPathElision
-	}
-
-	return masked
+	return MaskURL(c.WebhookURL)
 }