feat(core): add UserStorage with argon2 password hashing and username migration

[geoffjay]

Adds UserStorage with full CRUD operations, argon2id password hashing, lookup by username/email, set_active_organization, and paginated listing. Adds a second migration (m20260408_000002_add_username_to_users) to extend the users table with nullable username (unique-indexed) and active_organization_id columns. Updates all entity constructors and integration tests to include the new fields.

Closes #213

[geoffjay]

This change is part of the following stack:

• feat(core): scaffold agentd-core service #1058
  • feat(core): add entity module stubs, migration runner, and Storage wrapper #1059
    • feat(core): implement initial sea-orm migration for core service schema #1060
      • feat(core): define SeaORM User entity with has_many relations #1061
        • feat(core): define SeaORM Session entity with belongs_to User #1062
          • feat(core): define Organization and Membership entities with many-to-many relations #1063
            • feat(core): add SeaORM integration tests for entity relations #1064
              • feat(core): add OrganizationStorage and MembershipStorage #1066
                • feat(core): add UserStorage with argon2 password hashing and username migration #1067 ◀
                  • feat(core): add authentication endpoints with SQLite-backed sessions #1068
                  • feat(core): add TenantContext middleware and active organization endpoint #1069
                  • feat(core): add user and organization management API endpoints #1070
                  • feat(cli): add auth and org commands for core service #1071
                  • feat(core): add API gateway proxy for downstream service routing #1072

_{Change managed by git-spice.}