feat (user counert): add active_sessions table for counter

- Add active_sessions schema with session tracking
- Add index on last_activity for performance optimization
- Prepare database structure for live online user counter

Table structure:
- session_id (text, PK): unique session identifier
- last_activity (timestamp): tracks user's last activity time

Migration file: drizzle/0002_clean_martin_li.sql

Related to: (SP: 3) [Frontend + Backend] Track all active users and display live online counter on Home page

Author: YNazymko12

frontend/db/schema/index.ts

@@ -5,4 +5,5 @@ export * from './users';
 export * from './points';
 export * from './shop';
 export * from './emailVerificationTokens';
-export * from "./passwordResetTokens";
+export * from './passwordResetTokens';
+export * from './sessions';

frontend/db/schema/sessions.ts

@@ -0,0 +1,12 @@
+import { pgTable, text, timestamp, index } from 'drizzle-orm/pg-core';
+
+export const activeSessions = pgTable(
+  'active_sessions',
+  {
+    sessionId: text('session_id').primaryKey(),
+    lastActivity: timestamp('last_activity').notNull().defaultNow(),
+  },
+  (table) => ({
+    lastActivityIdx: index('active_sessions_last_activity_idx').on(table.lastActivity),
+  })
+);

frontend/drizzle/0002_clean_martin_li.sql

@@ -0,0 +1,6 @@
+CREATE TABLE "active_sessions" (
+	"session_id" text PRIMARY KEY NOT NULL,
+	"last_activity" timestamp DEFAULT now() NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX "active_sessions_last_activity_idx" ON "active_sessions" USING btree ("last_activity");