feat: Implement dispute flow with database schema and API endpoints

backend/sql/dispute-schema.sql

@@ -0,0 +1,28 @@
+CREATE TABLE IF NOT EXISTS disputes (
+  id BIGINT PRIMARY KEY AUTO_INCREMENT,
+  deal_id BIGINT NOT NULL,
+  opened_by_user_id BIGINT NOT NULL,
+  status ENUM('open','evidence','mediation','resolved','cancelled') NOT NULL DEFAULT 'open',
+  reason_code VARCHAR(64) NOT NULL,
+  summary TEXT NOT NULL,
+  requested_outcome VARCHAR(64) NOT NULL,
+  final_decision VARCHAR(64) NULL,
+  final_reason TEXT NULL,
+  decided_by_user_id BIGINT NULL,
+  decided_at TIMESTAMP NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  FOREIGN KEY (deal_id) REFERENCES deals(id),
+  FOREIGN KEY (opened_by_user_id) REFERENCES users(id)
+);
+
+CREATE TABLE IF NOT EXISTS dispute_events (
+  id BIGINT PRIMARY KEY AUTO_INCREMENT,
+  dispute_id BIGINT NOT NULL,
+  event_type VARCHAR(64) NOT NULL,
+  actor_user_id BIGINT NOT NULL,
+  payload_json JSON NOT NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+  FOREIGN KEY (dispute_id) REFERENCES disputes(id),
+  FOREIGN KEY (actor_user_id) REFERENCES users(id)
+);