Add referral system and user balance tracking

Introduce balance field to User model with Alembic migration. Add
referral menu, admin /refpay command, and deep link sharing. Update
Dockerfile to multi-stage build and add Postgres service to compose.
Fix deep link parsing logic and conditional proxy initialization.

.env.example

@@ -0,0 +1,29 @@
+# Telegram Bot Token (Required)
+# Get it from @BotFather on Telegram
+BOT_TOKEN=your_bot_token_here
+
+# Telegram Admin Group ID (Required)
+# The numeric ID of the group where admin notifications will be sent
+ADMIN_GROUP_ID=your_admin_group_id_here
+
+# Remnawave Configuration (Required)
+REMNAWAVE_URL=https://your-remnawave-instance.com
+REMNAWAVE_TOKEN=your_remnawave_api_token_here
+SUBSCRIPTION_URL=https://your-subscription-page.com
+
+# PostgreSQL Database Configuration
+# These variables are used by docker-compose to set up the database container
+POSTGRES_USER=malenia
+POSTGRES_PASSWORD=change_me_to_a_secure_password
+POSTGRES_DB=malenia_db
+
+# Proxy Configuration (Optional)
+# Leave empty if no proxy is needed
+PROXY=
+
+# Pricing and Limits Configuration (Optional - defaults provided in code)
+MIN_DEVICES=3
+MAX_DEVICES=12
+PER_DEVICE_COST=50
+WHITELIST_COST=100
+WHITELIST_DEVICE_THRESHOLD=6