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/web/view/templates/inbound.html

@@ -11,6 +11,9 @@ Off by default in Compose.</p>
 
 <div class="card">
   <h2>Forwarding</h2>
+  {{/* Adding and deleting a domain stay with the global role; a domain
+       administrator works with the inbound domains assigned to them. */}}
+  {{if .IsGlobal}}
   <form method="post" action="/inbound">
     <label for="name">Add inbound domain</label>
     <div class="input-row">
@@ -21,6 +24,7 @@ Off by default in Compose.</p>
     </div>
     {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
   </form>
+  {{end}}
   {{if .Domains}}
   <table>
     <thead>
@@ -34,7 +38,7 @@ Off by default in Compose.</p>
         <td class="muted">{{.Upstream}}</td>
         <td>{{.RcptLabel}}</td>
         <td><span class="st st-{{if eq .TLSMode "encrypt"}}ok{{else}}unknown{{end}}">{{.TLSLabel}}</span></td>
-        <td class="actions"><a class="danger" href="/inbound/{{.ID}}/delete">Delete</a></td>
+        <td class="actions">{{if $.IsGlobal}}<a class="danger" href="/inbound/{{.ID}}/delete">Delete</a>{{end}}</td>
       </tr>
       {{end}}
     </tbody>