Phase 5: reply drafts — template/LLM draft, edit, SMTP send (AUD-71)

Implements Phase 5 of AGENTS.md §11 (FR-8, FR-15):

- Message detail pane (GET /partials/message) with body, recommended
  action, reply identity, template picker, editable draft, and history.
- internal/reply: OmniRoute (OpenAI-compatible) chat client and a
  template-based draft generator. All non-JEV models go through
  OmniRoute (§10).
- internal/mailops: SMTP send over smtp.gmail.com:587 with STARTTLS and
  the app password, forcing IPv4 (§9); FR-15 reply identity (send as the
  arrived-at X-Pm-Original-To address, else "Sam Rolfe"); threading via
  In-Reply-To/References; -check-smtp safe verification; and a
  [Gmail]/Sent Mail thread check.
- Migration v5 stores messages.message_id_header and drafts.sent_at;
  mailfetch parses Message-ID.
- Send is user-triggered only: CSRF + UI confirm + confirm=yes guard,
  no auto-send, duplicate-send refused. A sent reply files the original
  under Replied-to/<address> and logs every step (FR-13).
- Tests for reply, mailops SMTP/identity/thread check, store drafts and
  migration v5, and the server draft/edit/send guards.
- docs/phase5-replies.md, README, .env.example, docker-compose updated.

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

internal/mailfetch/message.go

@@ -21,10 +21,13 @@ const maxBodyBytes = 1 << 20 // 1 MiB
 
 // ParsedMessage is the subset of an RFC822 message the agent stores.
 type ParsedMessage struct {
-	From          string
-	To            string
-	Subject       string
-	Date          time.Time
+	From    string
+	To      string
+	Subject string
+	Date    time.Time
+	// MessageID is the RFC822 Message-ID header, used for reply threading
+	// (Phase 5, FR-8).
+	MessageID     string
 	SourceAddress string
 	// SourceHeader records which header the source address came from, so FR-2
 	// decisions are auditable.
@@ -40,10 +43,11 @@ 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")),
+		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")),
 	}
 	pm.SourceAddress, pm.SourceHeader = SourceAddress(textproto.MIMEHeader(h))
 	pm.BodyText = extractBody(textproto.MIMEHeader(h), msg.Body)