fix(smtp): add missing headers to prevent spam classification

Add required SMTP headers to fix spam classification issues:
- Message-ID: generated from timestamp and from_address domain
- MIME-Version: 1.0 header for proper email formatting
- Content-Transfer-Encoding: 8bit for UTF-8 content

Fixes rspamd spam score from 10.42/10.00 (reject) to 4.80/10.00 (clean)
by resolving MISSING_MID (-2.50), MISSING_MIME_VERSION (-2.00),
and R_BAD_CTE_7BIT (-1.05) penalties.

Tested with mail-tester.com (10/10 score) and production deployment
on tiamat shows successful delivery to inbox instead of spam folder.

Related DAW/infrastruktur#35

src/smtp.lua

@@ -1,6 +1,6 @@
--- furt-lua/src/smtp.lua
+-- src/smtp.lua
 -- Universal SMTP implementation with SSL compatibility
--- Supports both luaossl (Arch/karl) and luasec (OpenBSD/walter)
+-- Supports both luaossl (Arch) and luasec (OpenBSD)
 -- Dragons@Work Digital Sovereignty Project
 
 local socket = require("socket")
@@ -304,6 +304,11 @@ function SMTP:send_email(to_address, subject, message, from_name)
         return cleanup_and_fail("DATA command failed: " .. response)
     end
 
+    -- Generate unique Message-ID
+    -- Extract domain from configured from_address
+    local hostname = self.from_address:match("@(.+)") or self.server
+    local message_id = string.format("<%d.%d@%s>", os.time(), math.random(10000), hostname)
+
     -- Build email message
     local display_name = from_name or "Furt Contact Form"
     local email_content = string.format(
@@ -311,7 +316,10 @@ function SMTP:send_email(to_address, subject, message, from_name)
         "To: <%s>\r\n" ..
         "Subject: %s\r\n" ..
         "Date: %s\r\n" ..
+        "Message-ID: %s\r\n" ..
+        "MIME-Version: 1.0\r\n" ..
         "Content-Type: text/plain; charset=UTF-8\r\n" ..
+        "Content-Transfer-Encoding: 8bit\r\n" ..
         "\r\n" ..
         "%s\r\n" ..
         ".",
@@ -320,6 +328,7 @@ function SMTP:send_email(to_address, subject, message, from_name)
         to_address,
         subject,
         os.date("%a, %d %b %Y %H:%M:%S %z"),
+        message_id,
         message
     )