feat: start mailbox evidence scanner

tests/fixtures/mailbox/hard_bounce.eml

@@ -0,0 +1,12 @@
+From: Mail Delivery Subsystem <mailer-daemon@example.net>
+To: sender@example.com
+Subject: Delivery Status Notification (Failure)
+Date: Tue, 02 Jun 2026 10:00:00 +0000
+Message-ID: <hard-bounce@example.net>
+Content-Type: text/plain; charset=utf-8
+
+Delivery failure.
+Final-Recipient: rfc822; missing@example.com
+Action: failed
+Status: 5.1.1
+Diagnostic-Code: smtp; 550 5.1.1 User unknown

tests/fixtures/mailbox/human_reply.eml

@@ -0,0 +1,8 @@
+From: Recipient <recipient@example.com>
+To: sender@example.com
+Subject: Re: Your notification
+Date: Tue, 02 Jun 2026 10:03:00 +0000
+Message-ID: <reply@example.com>
+Content-Type: text/plain; charset=utf-8
+
+Thanks, I received this and will review it today.

tests/fixtures/mailbox/out_of_office.eml

@@ -0,0 +1,9 @@
+From: Recipient <recipient@example.com>
+To: sender@example.com
+Subject: Auto-reply: Out of office
+Date: Tue, 02 Jun 2026 10:02:00 +0000
+Message-ID: <ooo@example.com>
+Auto-Submitted: auto-replied
+Content-Type: text/plain; charset=utf-8
+
+I am out of office until next week.

tests/fixtures/mailbox/soft_bounce.eml

@@ -0,0 +1,12 @@
+From: Mail Delivery Subsystem <mailer-daemon@example.net>
+To: sender@example.com
+Subject: Delivery temporarily delayed
+Date: Tue, 02 Jun 2026 10:01:00 +0000
+Message-ID: <soft-bounce@example.net>
+Content-Type: text/plain; charset=utf-8
+
+Delivery Status Notification.
+Final-Recipient: rfc822; full@example.com
+Action: delayed
+Status: 4.2.2
+Diagnostic-Code: smtp; 452 4.2.2 Mailbox full