feat(privacy): user blocklist with private chat enforcement

- add blocked_users table and migration
- add users API: block, unblock, list blocked users
- prevent private chat creation and private messaging when block relation exists
- add block/unblock action in private chat info panel

web/src/api/users.ts

@@ -24,3 +24,16 @@ export async function getUserById(userId: number): Promise<AuthUser> {
   const { data } = await http.get<AuthUser>(`/users/${userId}`);
   return data;
 }
+
+export async function listBlockedUsers(): Promise<UserSearchItem[]> {
+  const { data } = await http.get<UserSearchItem[]>("/users/blocked");
+  return data;
+}
+
+export async function blockUser(userId: number): Promise<void> {
+  await http.post(`/users/${userId}/block`);
+}
+
+export async function unblockUser(userId: number): Promise<void> {
+  await http.delete(`/users/${userId}/block`);
+}