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>

CHANGELOG.md

@@ -38,6 +38,16 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   `user_inbound_domains` and the per-user `all_domains` /
   `all_inbound_domains` flags are in the schema for the inbound delegation
   that follows. Details: `docs/schema-migrations.md`.
+- panel-redesign (stage 1): inbound domains are delegated to domain
+  administrators, separately from outbound. The user form has two lists, each
+  with *All* (which includes domains added later; with it ticked the rows
+  under it are ignored); a domain administrator needs at least one assignment
+  on either. Such a user sees the inbound domains assigned to them — the list
+  is filtered in the query — and can change their upstream and recipients;
+  adding and deleting an inbound domain stays with the global role. Another
+  tenant's domain, a missing one and a user with no inbound reach all get the
+  same 404. A menu group with nothing assigned is absent, and `/` leads an
+  inbound-only administrator to Inbound.
 
 ### Changed