Adds users.auto_approve_requests boolean default false (the #355
sub-feature surface; the request-flow handler that honors this
flag is U2.5 follow-up work).
Four new sqlc queries for the U2 admin endpoints:
- CreateUserAdmin: admin-driven user creation, accepts all five
fields explicitly.
- DeleteUser: hard delete; schema's ON DELETE CASCADE foreign
keys handle plays/likes/sessions cleanup. Last-admin guard
lives in the HTTP layer (next task).
- ResetUserPassword: admin sets a new hashed password without
knowing the old one.
- UpdateUserAutoApprove: toggles the new boolean.
Self-service equivalents (knows-current-password change, etc.)
are U3 work and use distinct queries.
