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/core.go

@@ -0,0 +1,58 @@
+// Package core holds OpenMail's domain services: inboxes, messages, threads,
+// drafts, and the inbound ingest path (parse → thread → store). It is the layer
+// the API and MCP server call, and it implements mail.InboundSink so any
+// MailBackend can deliver into it.
+package core
+
+import (
+	"context"
+	"errors"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/karti-ai/openmail/internal/store"
+)
+
+// ErrNotFound is returned when a requested resource does not exist.
+var ErrNotFound = errors.New("core: not found")
+
+type Service struct {
+	pool *pgxpool.Pool
+}
+
+func New(st *store.Store) *Service { return &Service{pool: st.Pool} }
+
+// Ping verifies the database connection (used by the health endpoint).
+func (s *Service) Ping(ctx context.Context) error { return s.pool.Ping(ctx) }
+
+// EnsureDefaultPod returns the id of the singleton "default" pod, creating it if
+// absent. Until DB-backed api_keys carry a pod_id, the API operates within this
+// one tenant. Idempotent.
+func (s *Service) EnsureDefaultPod(ctx context.Context) (string, error) {
+	var id string
+	err := s.pool.QueryRow(ctx, `SELECT id::text FROM pods WHERE name = 'default'`).Scan(&id)
+	if err == nil {
+		return id, nil
+	}
+	if !errors.Is(err, pgx.ErrNoRows) {
+		return "", err
+	}
+	err = s.pool.QueryRow(ctx,
+		`INSERT INTO pods (name) VALUES ('default')
+		 ON CONFLICT DO NOTHING
+		 RETURNING id::text`).Scan(&id)
+	if errors.Is(err, pgx.ErrNoRows) {
+		// Lost a race; read the row the other writer created.
+		err = s.pool.QueryRow(ctx, `SELECT id::text FROM pods WHERE name = 'default'`).Scan(&id)
+	}
+	return id, err
+}
+
+// nullStr maps an optional string to a value usable as a nullable SQL arg.
+func nullStr(s *string) any {
+	if s == nil {
+		return nil
+	}
+	return *s
+}