chore: add migration for api key rate limit fields

backend/migrations/064_add_api_key_rate_limits.sql

+-- Add rate limit fields to api_keys table
+-- Rate limit configuration (0 = unlimited)
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS rate_limit_5h decimal(20,8) NOT NULL DEFAULT 0;
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS rate_limit_1d decimal(20,8) NOT NULL DEFAULT 0;
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS rate_limit_7d decimal(20,8) NOT NULL DEFAULT 0;
+
+-- Rate limit usage tracking
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS usage_5h decimal(20,8) NOT NULL DEFAULT 0;
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS usage_1d decimal(20,8) NOT NULL DEFAULT 0;
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS usage_7d decimal(20,8) NOT NULL DEFAULT 0;
+
+-- Window start times (nullable)
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS window_5h_start timestamptz;
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS window_1d_start timestamptz;
+ALTER TABLE api_keys ADD COLUMN IF NOT EXISTS window_7d_start timestamptz;