feat: Group chat and push notifications

- Add group chat tables (chat_groups, chat_group_members, group_messages)
- Add push subscriptions table for web push notifications
- Add group service, controller, routes
- Add push service, controller, routes
- Integrate push notifications with todos, messages, group messages

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/db/schema.js

@@ -337,3 +337,43 @@ export const messages = pgTable('messages', {
   createdAt: timestamp('created_at').defaultNow().notNull(),
   updatedAt: timestamp('updated_at').defaultNow().notNull(),
 });
+
+// Chat Groups table - skupinové chaty
+export const chatGroups = pgTable('chat_groups', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  name: text('name').notNull(),
+  createdById: uuid('created_by_id').references(() => users.id, { onDelete: 'set null' }),
+  createdAt: timestamp('created_at').defaultNow().notNull(),
+  updatedAt: timestamp('updated_at').defaultNow().notNull(),
+});
+
+// Chat Group Members - členovia skupiny
+export const chatGroupMembers = pgTable('chat_group_members', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  groupId: uuid('group_id').references(() => chatGroups.id, { onDelete: 'cascade' }).notNull(),
+  userId: uuid('user_id').references(() => users.id, { onDelete: 'cascade' }).notNull(),
+  joinedAt: timestamp('joined_at').defaultNow().notNull(),
+}, (table) => ({
+  uniqueMember: unique('chat_group_member_unique').on(table.groupId, table.userId),
+}));
+
+// Group Messages - správy v skupinách
+export const groupMessages = pgTable('group_messages', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  groupId: uuid('group_id').references(() => chatGroups.id, { onDelete: 'cascade' }).notNull(),
+  senderId: uuid('sender_id').references(() => users.id, { onDelete: 'set null' }),
+  content: text('content').notNull(),
+  createdAt: timestamp('created_at').defaultNow().notNull(),
+});
+
+// Push Subscriptions - web push notifikácie
+export const pushSubscriptions = pgTable('push_subscriptions', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  userId: uuid('user_id').references(() => users.id, { onDelete: 'cascade' }).notNull(),
+  endpoint: text('endpoint').notNull(),
+  p256dh: text('p256dh').notNull(),
+  auth: text('auth').notNull(),
+  createdAt: timestamp('created_at').defaultNow().notNull(),
+}, (table) => ({
+  uniqueEndpoint: unique('push_subscription_endpoint_unique').on(table.userId, table.endpoint),
+}));