Add admin user management with is_active enforcement

Admin users page at /admin/users with paginated user list showing name,
email, team counts, role, join date, and active status toggle. Inactive
users are blocked from all authenticated endpoints immediately via DB
check in JWT middleware. OAuth login errors now show human-readable
messages on the login page.

db/migrations/20260414213729_add_user_active_deleted.sql

@@ -0,0 +1,7 @@
+-- +goose Up
+ALTER TABLE users ADD COLUMN is_active BOOLEAN NOT NULL DEFAULT TRUE;
+ALTER TABLE users ADD COLUMN deleted_at TIMESTAMPTZ;
+
+-- +goose Down
+ALTER TABLE users DROP COLUMN deleted_at;
+ALTER TABLE users DROP COLUMN is_active;