Fix 13 findings from adversarial milestone-1 review

Blockers in the ingest path (all funnel real mail in milestone 2):
- H1: coerce body bytes to valid UTF-8 (ToValidUTF8) — non-UTF-8/mid-rune cuts
  no longer abort the ingest tx and drop the message.
- H2: EnsurePart's recoverable parse error is non-fatal — proceed with the
  guaranteed-usable Part so messy real-world mail is stored, not rejected.
- H3: extractBodies descends into message/rfc822 (Part.Message via
  SetMessageReaderAt) — forwarded/bounce bodies no longer lost.
- H4: GetMessage/GetThread/GetThreadMessages scoped to inbox_id — no cross-inbox
  access; reply no longer a confused deputy.

Hardening:
- M1: /healthz no longer leaks DB error to unauthenticated callers.
- M2: all DB errors funnel through handleErr; malformed UUID -> 404, dup -> 409,
  internal errors no longer echo the driver string.
- M3: index messages(inbox_id, message_id_hdr) for thread resolution.
- M4: pods UNIQUE(name) + ON CONFLICT (name) — no duplicate default pods.
- L1: skip empty-User/Host addresses (no literal "@").
- L2: skip attachment-disposition parts when picking the body.
- L3: case-insensitive, trimmed 'Re:' detection.

Verified e2e vs Postgres 16: latin1 body stored valid UTF-8; rfc822-only body
extracted; cross-inbox 404; malformed UUID 404; dup 409; threading regression OK.

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

internal/core/thread.go

@@ -45,8 +45,11 @@ func (s *Service) ListThreads(ctx context.Context, inboxID string) ([]Thread, er
 	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))
+// GetThread is scoped to the inbox: a thread id belonging to another inbox
+// returns ErrNotFound.
+func (s *Service) GetThread(ctx context.Context, inboxID, id string) (Thread, error) {
+	t, err := scanThread(s.pool.QueryRow(ctx,
+		`SELECT `+threadCols+` FROM threads WHERE id = $1 AND inbox_id = $2`, id, inboxID))
 	if errors.Is(err, pgx.ErrNoRows) {
 		return Thread{}, ErrNotFound
 	}