Phase 6: rules editor + unsubscribe detection (AUD-72)

- FR-11 rules editor: plain-text area at /rules, saved and versioned
  (SaveRules appends a version; restore re-saves an old version), parse
  errors surfaced, one-click re-classify. The reply model now reads the
  rules document too (FR-11).
- FR-7 unsubscribe: parse and store List-Unsubscribe / List-Unsubscribe-Post
  (migration v6), detect email and web links (internal/unsubscribe), a
  domain-level ignore list, and an explicit record of what was marked. No
  code path contacts an unsubscribe link; acting on one needs Sam's approval.
- Migration v6 is additive; unsubscribe gains url/updated_at.
- Tests: detector, parser, store migration/records/candidates, rules editor,
  unsubscribe handlers, message-detail rendering.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/mailfetch/message.go

@@ -27,8 +27,13 @@ type ParsedMessage struct {
 	Date    time.Time
 	// MessageID is the RFC822 Message-ID header, used for reply threading
 	// (Phase 5, FR-8).
-	MessageID     string
-	SourceAddress string
+	MessageID string
+	// ListUnsubscribe and ListUnsubscribePost are the RFC 2369/8058 headers that
+	// name the unsubscribe targets (Phase 6, FR-7). They are stored raw so the
+	// detector can run later without a refetch.
+	ListUnsubscribe     string
+	ListUnsubscribePost string
+	SourceAddress       string
 	// SourceHeader records which header the source address came from, so FR-2
 	// decisions are auditable.
 	SourceHeader string
@@ -43,11 +48,13 @@ func ParseMessage(raw []byte) (*ParsedMessage, error) {
 	}
 	h := msg.Header
 	pm := &ParsedMessage{
-		From:      decodeHeader(h.Get("From")),
-		To:        decodeHeader(h.Get("To")),
-		Subject:   decodeHeader(h.Get("Subject")),
-		Date:      parseDate(h.Get("Date")),
-		MessageID: strings.TrimSpace(h.Get("Message-Id")),
+		From:                decodeHeader(h.Get("From")),
+		To:                  decodeHeader(h.Get("To")),
+		Subject:             decodeHeader(h.Get("Subject")),
+		Date:                parseDate(h.Get("Date")),
+		MessageID:           strings.TrimSpace(h.Get("Message-Id")),
+		ListUnsubscribe:     strings.TrimSpace(h.Get("List-Unsubscribe")),
+		ListUnsubscribePost: strings.TrimSpace(h.Get("List-Unsubscribe-Post")),
 	}
 	pm.SourceAddress, pm.SourceHeader = SourceAddress(textproto.MIMEHeader(h))
 	pm.BodyText = extractBody(textproto.MIMEHeader(h), msg.Body)