feat(headers): add parsing for Stalwart spam headers

Example of headers created by Stalwart below

X-Spam-Result: TRUSTED_DOMAIN (-7.00),
	PROB_HAM_LOW (-2.00),
	RBL_SENDERSCORE_REPUT_9 (-1.00),
	DMARC_POLICY_ALLOW (-0.50),
	RCVD_DKIM_ARC_DNSWL_MED (-0.50)
X-Spam-Score: ham, score=-5.60, avg_confidence=0.26
X-Spam-Status: No

Only need to add small tweak by detecting spam|ham as well as Yes|No
The most useful header is X-Spam-Score, so we make sure to parse that
before X-Spam-Status

lib/email-headers.ts

@@ -118,7 +118,8 @@ export function parseAuthenticationResults(header: string): AuthenticationResult
  */
 export function parseSpamScore(header: string): { score: number; status: string } | null {
   // Try X-Spam-Status format: "No, score=-0.25"
-  const statusMatch = header.match(/^(Yes|No),?\s+score=([-\d.]+)/i);
+  // And try X-Spam-Score format (Stalwart): "ham, score=-0.25"
+  const statusMatch = header.match(/^(Yes|No|spam|ham),?\s+score=([-\d.]+)/i);
   if (statusMatch) {
     return {
       status: statusMatch[1].toLowerCase(),