panel-redesign: inbound is delegated per user, separately from outbound

Stage 1, step 2 (plan § Who sees what).

Store
  - Reach{AllDomains, DomainIDs, AllInbound, InboundDomainIDs} is what a
    domain user is assigned. CreateUser takes it; SetUserReach / ClearUserReach
    replace SetUserDomains / ClearUserDomains and write both directions in one
    transaction. A domain user with nothing on either list is refused
    (ErrEmptyReach). With All set the rows of that direction are not stored,
    so clearing All later brings nothing stale back.
  - ListDomainsForUser honours all_domains; ListInboundDomainsForUser is its
    inbound twin. Both filter in the query.

Authorization
  - auth.Principal carries the whole reach: CanAccessDomain (now also true
    with All), CanAccessInboundDomain, HasOutbound, HasInbound. An id or a
    name on one list opens nothing on the other.
  - Inbound handlers in three strengths: requireInbound (the list, filtered),
    requireInboundDomain (one domain: page, DNS re-check, upstream,
    recipients), requireInboundGlobal (add, delete confirm, delete). All
    answer the same 404.
  - domainIDSet and the DMARC report pages go through the principal instead
    of looking at p.Domains themselves.

Panel (old templates, until stage 2)
  - User form: two lists with All; inbound is offered only where the feature
    is on, and saving the form with it off keeps the user's inbound reach.
    Promotion to global drops the reach.
  - Menu: a group with nothing assigned is absent; / sends an inbound-only
    administrator to Inbound. The inbound pages hide add / delete from a
    domain administrator.

Tests: every route of one inbound domain against five kinds of refused
request (another tenant's id, a missing id, outbound-only reach, an outbound
domain with the same id, one with the same name, no principal) with the 404
bodies compared; add/delete as every non-global user; the filtered list
including a domain added later; thirteen outbound routes against another
tenant's domain and application id; the user form; the reach in the store;
the principal matrix; the menu; the home redirect. Removing the access check
or the query filter makes the tests fail (checked by hand).

go test ./...  green except the two Windows-only failures in internal/domain
and internal/logtail that predate this work.

Roadmap 12/27.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/domains.go

@@ -80,16 +80,18 @@ func (s *Store) ListDomains() ([]Domain, error) {
 	return out, rows.Err()
 }
 
-// ListDomainsForUser returns domains assigned to userID with application counts,
-// ordered by name.
+// ListDomainsForUser returns the sending domains a domain user reaches, with
+// application counts, ordered by name: the assigned ones, or every domain when
+// the user's All flag is set — which is how a domain added later is included.
+// The filter is in the query, so a caller cannot forget it.
 func (s *Store) ListDomainsForUser(userID int64) ([]Domain, error) {
 	rows, err := s.db.Query(`
 		SELECT d.id, d.name, d.dkim_selector, d.dmarc_rua, d.dmarc_rua_user_id, d.created_at,
 		       (SELECT COUNT(*) FROM applications a WHERE a.domain_id = d.id)
 		FROM domains d
-		INNER JOIN user_domains ud ON ud.domain_id = d.id
-		WHERE ud.user_id = ?
-		ORDER BY d.name`, userID)
+		WHERE EXISTS (SELECT 1 FROM users u WHERE u.id = ? AND u.all_domains = 1)
+		   OR EXISTS (SELECT 1 FROM user_domains ud WHERE ud.user_id = ? AND ud.domain_id = d.id)
+		ORDER BY d.name`, userID, userID)
 	if err != nil {
 		return nil, fmt.Errorf("list domains for user: %w", err)
 	}