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/ssrf.go

@@ -92,7 +92,12 @@ func ValidateTargetURL(
 ) error {
 	parsed, err := url.Parse(targetURL)
 	if err != nil {
-		return fmt.Errorf("invalid URL: %w", err)
+		// url.Parse embeds the whole URL in its error, and
+		// this one is logged and shown; mask it. Every other
+		// branch below reports only the hostname.
+		return fmt.Errorf(
+			"invalid URL: %w", maskURLError(err),
+		)
 	}
 
 	err = validateScheme(parsed.Scheme)