Implement backend for blackjack game with Discord authentication, database integration, and WebSocket support

schema.sql

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+CREATE TABLE IF NOT EXISTS users (
+  id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+  discord_id VARCHAR(32) NOT NULL UNIQUE,
+  username VARCHAR(100) NOT NULL,
+  avatar VARCHAR(128) NOT NULL DEFAULT '',
+  balance INT NOT NULL DEFAULT 0,
+  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS deposits (
+  id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+  user_id BIGINT UNSIGNED NOT NULL,
+  amount INT NOT NULL,
+  stripe_payment_intent_id VARCHAR(128) NOT NULL,
+  status VARCHAR(32) NOT NULL DEFAULT 'created',
+  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  CONSTRAINT fk_deposits_user_id FOREIGN KEY (user_id) REFERENCES users(id)
+);
+
+CREATE INDEX idx_deposits_stripe ON deposits (stripe_payment_intent_id);