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/queries/users.sql

@@ -43,3 +43,26 @@ SELECT id, email FROM users WHERE email LIKE $1 || '%' ORDER BY email LIMIT 10;
 
 -- name: UpdateUserName :exec
 UPDATE users SET name = $2, updated_at = NOW() WHERE id = $1;
+
+-- name: ListUsersAdmin :many
+SELECT
+    u.id,
+    u.email,
+    u.name,
+    u.is_admin,
+    u.is_active,
+    u.created_at,
+    (SELECT COUNT(*) FROM users_teams ut WHERE ut.user_id = u.id)::int AS teams_joined,
+    (SELECT COUNT(*) FROM users_teams ut WHERE ut.user_id = u.id AND ut.role = 'owner')::int AS teams_owned
+FROM users u
+WHERE u.deleted_at IS NULL
+ORDER BY u.created_at DESC
+LIMIT $1 OFFSET $2;
+
+-- name: CountUsersAdmin :one
+SELECT COUNT(*)::int AS total
+FROM users
+WHERE deleted_at IS NULL;
+
+-- name: SetUserActive :exec
+UPDATE users SET is_active = $2, updated_at = NOW() WHERE id = $1;