feat(dev): add scripts/dev-seed to populate sample test data

New scripts/dev-seed.sh (+ .ps1), run after dev-server-init: seeds the
test server with Users (alice, bob, carol — spanning User/Admin roles,
group membership, an alias), Groups (engineering, marketing), Mailing
Lists (newsletter, support), and custom Roles (Support Agent,
Read-only Auditor), so each admin panel screen has real data to test
against instead of an empty list.

Idempotent (skips if 'alice' already exists), documented as workflow
step 3 in DEVELOPMENT.md (renumbering the steps after it) and listed
in AGENTS.md. Verified end-to-end with both scripts against a fresh
server, including idempotency and the resulting UI (Accounts, Groups,
Mailing Lists, Roles all populated as expected).

.gitignore

@@ -28,6 +28,8 @@ scripts/*
 !scripts/dev-token.sh
 !scripts/dev-server-init.ps1
 !scripts/dev-server-init.sh
+!scripts/dev-seed.ps1
+!scripts/dev-seed.sh
 *.md
 !README.md
 !CHANGELOG.md