panel-redesign: security review of 2.0 - sessions belong to a user by id

Stage 3, step 2. Reviewed by Fable (read-only), fixed by Sonnet with a
failing test before each fix, checked by Opus. No Critical or High
finding: route authorization, tenant isolation by id, the /server/
subtree guard, output encoding and the origin check were found sound.

Fixed
  - M1: a session was keyed by user name and had no link to the user. A
    deleted or renamed user's cookie stayed alive (renewed on every hit,
    before the user was looked up) and became a session of whoever was
    later created under that name, with that account's role. The
    baseline now has sessions.user_id REFERENCES users(id) ON DELETE
    CASCADE; the principal is resolved by id and a session is renewed
    only after its user was found. RenameSession is gone.
  - M2: any user's password change deleted every other user's sessions.
    It ends that user's other sessions only.
  - L1: a password set on the user form ends that user's sessions (an
    administrator editing their own account keeps the current one).
  - L2: store, saslpasswd2 and Postfix reload errors were shown verbatim
    on the application form, the inbound upstream / recipients forms and
    the limit recalculation - pages a domain administrator reaches.
    Validation errors (app.ValidationError, inbound.ValidationError,
    store.ErrNotAutoRateLimit) keep their text; anything else is logged
    and shown as "Could not save ... Check the logs and try again."
  - L3: store.ListDMARCReports treated an empty domain list as "all". It
    takes a scope now (AllDomains or a list; an empty list is no rows),
    and the DMARC hub is open to the domain role as the plan says: the
    reports of the assigned domains, without the instance-wide ingest
    box. The menu entry was already shown to them and led to a 404.
  - I1: /static/ answered with a listing of the embedded assets. A path
    that is not a known asset is a 404.

Consequences
  - The global DMARC hub lists reports of domains that were deleted
    since, too.
  - dev/migrate1x (not in the tree) no longer carries sessions over:
    everyone signs in again after the transfer.
  - docs/schema-migrations.md: the sessions row.

Recorded, not changed - for the owner:
  - L4: the report-address select names the followed user and their
    address to every administrator of the domain, by default the global
    administrator's login name. The plan asks for it.
  - L5: a delegated inbound upstream may be any host and port, loopback
    and private ranges included - within the trusted-operator model; to
    be listed among the accepted risks in security.md.
  - L6: Account's profile is saved without the password; becomes a
    requirement once password-reset mails to users.email.
  - Still echoing an error's text: backup and domain import (global
    only), the domain form's validation messages.

Not checked by the review: anything needing the container (e2e, real
Postfix / DNS), CSP behaviour in a live browser.

  go test -count=1 ./internal/web/... ./internal/store/... ./internal/app/...
    ./internal/inbound/... ./internal/dmarc/... ./cmd/panel
      ok  (all eleven packages; dev/migrate1x also ok)

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

internal/web/handlers/handlers_users.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/mixeme/selfpost/internal/store"
+	"github.com/mixeme/selfpost/internal/web/auth"
 	"github.com/mixeme/selfpost/internal/web/validate"
 	"github.com/mixeme/selfpost/internal/web/view"
 	"golang.org/x/crypto/bcrypt"
@@ -310,6 +311,9 @@ func (h *Handlers) submitUserUpdate(w http.ResponseWriter, r *http.Request, u st
 		return
 	}
 	h.resyncAfterEmailChange(u, email)
+	if password != "" {
+		h.endSessionsAfterPasswordSet(r, p, u)
+	}
 
 	if role != u.Role {
 		if err := h.store.SetUserRole(u.ID, role); err != nil {
@@ -335,6 +339,19 @@ func (h *Handlers) submitUserUpdate(w http.ResponseWriter, r *http.Request, u st
 	http.Redirect(w, r, "/server/users?done=updated", http.StatusSeeOther)
 }
 
+// endSessionsAfterPasswordSet signs out the logins a user had under the old
+// password when an administrator sets a new one on the user form. An
+// administrator who sets their own password keeps the browser they are using.
+func (h *Handlers) endSessionsAfterPasswordSet(r *http.Request, by auth.Principal, u store.User) {
+	if u.ID == by.ID {
+		if token, ok := h.auth.SessionToken(r); ok {
+			h.auth.DestroyOtherSessions(u.ID, token)
+			return
+		}
+	}
+	h.auth.DestroyUserSessions(u.ID)
+}
+
 // HandleUserDeleteConfirm shows the cascade warning before a panel user is
 // removed — the same pattern as HandleDeleteConfirm for domains, so a single
 // mis-click on Delete cannot remove a user (P3, code-review.md).