Milestone 1: core services + Mox ingest + wired v0 API

- internal/mail: MailBackend interface (Send/Start/Capabilities) + NullBackend.
- internal/core: inbox/message/thread services over pgx; default-pod bootstrap.
- IngestRaw: real mox/message parse → body extraction (text/html walk) →
  quote-stripping (extracted_text) → reference-based threading → store, all in
  one tx; emits message.received event; populates FTS tsvector.
- core implements mail.InboundSink (Deliver) so any backend feeds the same path.
- internal/api: v0 routes wired live (was 501) — create/list/get inbox, ingest,
  list/get messages, list/get thread(+messages), send/reply (503 via NullBackend).
- cmd/openmail serve: builds core + NullBackend, ensures default pod.

Verified e2e vs Postgres 16: inbox create; ingest original+reply → same thread
via In-Reply-To/References; extracted_text strips quoted history; thread detail
ordered; FTS matches 'invoice'; events written; send → honest 503. vet clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/core/thread.go

@@ -0,0 +1,54 @@
+package core
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+)
+
+type Thread struct {
+	ID            string    `json:"id"`
+	InboxID       string    `json:"inbox_id"`
+	Subject       *string   `json:"subject,omitempty"`
+	LastMessageID *string   `json:"last_message_id,omitempty"`
+	MessageCount  int       `json:"message_count"`
+	Labels        []string  `json:"labels"`
+	CreatedAt     time.Time `json:"created_at"`
+	UpdatedAt     time.Time `json:"updated_at"`
+}
+
+const threadCols = `id::text, inbox_id::text, subject, last_message_id::text, message_count, labels, created_at, updated_at`
+
+func scanThread(row pgx.Row) (Thread, error) {
+	var t Thread
+	err := row.Scan(&t.ID, &t.InboxID, &t.Subject, &t.LastMessageID, &t.MessageCount, &t.Labels, &t.CreatedAt, &t.UpdatedAt)
+	return t, err
+}
+
+func (s *Service) ListThreads(ctx context.Context, inboxID string) ([]Thread, error) {
+	rows, err := s.pool.Query(ctx,
+		`SELECT `+threadCols+` FROM threads WHERE inbox_id = $1 ORDER BY updated_at DESC`, inboxID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := []Thread{}
+	for rows.Next() {
+		t, err := scanThread(rows)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, t)
+	}
+	return out, rows.Err()
+}
+
+func (s *Service) GetThread(ctx context.Context, id string) (Thread, error) {
+	t, err := scanThread(s.pool.QueryRow(ctx, `SELECT `+threadCols+` FROM threads WHERE id = $1`, id))
+	if errors.Is(err, pgx.ErrNoRows) {
+		return Thread{}, ErrNotFound
+	}
+	return t, err
+}