Mask the target URL in delivery errors, SSRF logs and log page data (closes #118)

A delivery target URL is itself a credential: a Slack incoming
webhook URL is a bearer token. Three paths still reproduced it
in full.

Transport failures were the worst of them. net/http embeds the
request URL in every *url.Error it returns, so any DNS, TLS,
timeout or dial failure wrote the whole webhook URL into
DeliveryResult.Error — on disk, in the per-webhook database,
behind a json tag that a REST API would serialize.

maskURL moves to url_mask.go and is exported as MaskURL, and
maskURLError joins it: it rebuilds the *url.Error with the URL
masked, keeping the operation and the wrapped cause, so a
refused connection still reads differently from a DNS failure
or a timeout and errors.Is/As/Timeout still work. It is applied
where the errors are raised — executeHTTPRequest, shared by the
Slack and HTTP targets, and the request-construction paths — so
downstream wrapping is safe by construction. url.Parse embeds
the URL too, so ValidateTargetURL's parse branch gets the same
treatment; its error is logged and shown.

The SSRF rejection log now records only the masked URL, and
loadTargetMap hands the event log page TargetViews and a
delivery projection instead of raw target rows, so the stored
config blob has no path to that template either.

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)
 }