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/url_mask.go

@@ -0,0 +1,61 @@
+package delivery
+
+import (
+	"errors"
+	"net/url"
+)
+
+// urlPathElision stands in for a URL's elided path.
+const urlPathElision = "/..."
+
+// MaskURL renders a URL as scheme plus host with everything
+// that can carry a secret removed. A delivery target URL is
+// itself a credential — a Slack incoming webhook URL is a
+// bearer token — so the path, query and userinfo are never
+// reproduced, in a page, a log line or a stored error. A URL
+// that does not parse into a scheme and host yields the
+// neutral placeholder, never the raw string.
+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
+}
+
+// maskURLError strips the credential from an error raised
+// against a request URL. The net/http and net/url packages
+// embed the full request URL in every *url.Error they return,
+// so an unmodified transport error persisted into
+// DeliveryResult.Error writes the credential to disk.
+//
+// The masked error keeps the operation and the wrapped cause,
+// so a DNS failure still reads differently from a refused
+// connection, a TLS handshake failure or a timeout, and Is,
+// As, Timeout and Temporary keep working on it. Only the
+// path, query and userinfo of the URL are dropped. Errors
+// that carry no URL are returned unchanged.
+//
+// Call it where the error is raised, before any wrapping: it
+// replaces the *url.Error itself, so any context wrapped
+// around it first would be discarded.
+func maskURLError(err error) error {
+	var urlErr *url.Error
+	if !errors.As(err, &urlErr) {
+		return err
+	}
+
+	return &url.Error{
+		Op:  urlErr.Op,
+		URL: MaskURL(urlErr.URL),
+		Err: urlErr.Err,
+	}
+}