Harden operator-set target headers (closes #233)

Three findings from the review of the per-target request headers
feature.

Configured headers no longer follow a redirect off the origin the
target names. net/http withholds only Authorization and Cookie
across a host change, so an operator's X-Api-Key or PRIVATE-TOKEN
would follow a 302 to a host they never configured. Redirects are
still followed — refusing them would break every destination that
legitimately redirects and would record the 3xx as the delivery's
result — but a hop to another host, another port, or down from
https to http drops every header the target configured. The shared
SSRF-safe transport is kept on that client, so each hop is still
dialled through the private-IP guard.

Trailer joins the reserved names. net/http strips it from the
request it writes, so a configured one was accepted, stored, and
provably never sent.

The invalid-header-name error no longer quotes the text before the
first colon. That text is only a name if it parses as one; when it
does not, a pasted value whose own colon split the line put half a
token into a 400 body. TestParseTargetHeaders_ErrorsNeverQuoteAValue
asserted this invariant while only exercising the after-the-colon
case, and now covers the before-the-colon one.

README documents the http target's config keys, the 300-second
timeout ceiling, the reserved-header list and the redirect
behaviour; the edit form's hint gains Trailer and the redirect note.

internal/delivery/target_headers.go

@@ -65,6 +65,11 @@ func isReservedTargetHeader(name string) bool {
 		// the configured headers, so a configured one would always
 		// be overwritten.
 		return true
+	case "Trailer":
+		// net/http strips Trailer from the request it writes
+		// (reqWriteExcludeHeader), so a configured one is accepted
+		// and stored and then provably never reaches the wire.
+		return true
 	default:
 		return false
 	}
@@ -118,9 +123,11 @@ func parseHeaderLine(line string) (string, string, error) {
 
 	rawName = strings.TrimSpace(rawName)
 	if !validHeaderName(rawName) {
-		return "", "", fmt.Errorf(
-			"%w: %q", errHeaderNameInvalid, rawName,
-		)
+		// Quotes nothing. The text before the first colon is only
+		// a name if it parses as one; when it does not, it is as
+		// likely to be a pasted value whose own colon split the
+		// line, and half of a token would be echoed into the 400.
+		return "", "", errHeaderNameInvalid
 	}
 
 	name := http.CanonicalHeaderKey(rawName)