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/layout_legacy.html

@@ -70,20 +70,26 @@
        the column: the mark, the panel's pages, and the session last. The mark
        goes to /status, the page the panel opens on — named outright rather
        than as "/", which is only a redirect to it. */}}
-  <a class="brand" href="{{if .IsGlobal}}/status{{else}}/domains{{end}}"><img src="/static/logo-compact.svg" width="220" height="100" alt="SelfPost"></a>
+  <a class="brand" href="{{if .IsGlobal}}/status{{else if .HasOutbound}}/domains{{else}}/inbound{{end}}"><img src="/static/logo-compact.svg" width="220" height="100" alt="SelfPost"></a>
   <div class="links">
     {{if .IsGlobal}}
     {{if eq .Active "status"}}<span aria-current="page">{{template "icon-status"}}Status</span>{{else}}<a href="/status">{{template "icon-status"}}Status</a>{{end}}
     {{end}}
+    {{/* A group with nothing assigned is absent from that user's menu: the
+         sending pages need outbound reach, Inbound needs inbound reach. */}}
+    {{if or .IsGlobal .HasOutbound}}
     {{if eq .Active "domains"}}<span aria-current="page">{{template "icon-domains"}}Domains</span>{{else}}<a href="/domains">{{template "icon-domains"}}Domains</a>{{end}}
-    {{if and .IsGlobal .InboundEnabled}}
+    {{end}}
+    {{if and .InboundEnabled (or .IsGlobal .HasInbound)}}
     {{if eq .Active "inbound"}}<span aria-current="page">{{template "icon-inbound"}}Inbound</span>{{else}}<a href="/inbound">{{template "icon-inbound"}}Inbound</a>{{end}}
     {{end}}
-    {{if .DMARCEnabled}}
+    {{if and .DMARCEnabled (or .IsGlobal .HasOutbound)}}
     {{if eq .Active "dmarc"}}<span aria-current="page">{{template "icon-dmarc"}}DMARC</span>{{else}}<a href="/dmarc">{{template "icon-dmarc"}}DMARC</a>{{end}}
     {{end}}
     {{if eq .Active "help"}}<span aria-current="page">{{template "icon-help"}}Help</span>{{else}}<a href="/help">{{template "icon-help"}}Help</a>{{end}}
+    {{if or .IsGlobal .HasOutbound}}
     {{if eq .Active "deliveries"}}<span aria-current="page">{{template "icon-deliveries"}}Deliveries</span>{{else}}<a href="/deliveries">{{template "icon-deliveries"}}Deliveries</a>{{end}}
+    {{end}}
     {{if .IsGlobal}}
     {{if eq .Active "mail_queue"}}<span aria-current="page">{{template "icon-mail-queue"}}Mail queue</span>{{else}}<a href="/mail-queue">{{template "icon-mail-queue"}}Mail queue</a>{{end}}
     {{if eq .Active "system_log"}}<span aria-current="page">{{template "icon-system-log"}}System log</span>{{else}}<a href="/system-log">{{template "icon-system-log"}}System log</a>{{end}}