feat(billing): record subscription-covered token usage in usage records

Write h5_usage_records when plan quota fully covers a session bill so
WeChat and H5 consumption stays visible in admin usage history.

Co-authored-by: Cursor <cursoragent@cursor.com>

schema.sql

@@ -511,6 +511,7 @@ CREATE TABLE IF NOT EXISTS h5_usage_records (
   output_tokens INT NOT NULL DEFAULT 0,
   cost_cents BIGINT NOT NULL,
   balance_after_cents BIGINT NOT NULL,
+  billing_source VARCHAR(16) NOT NULL DEFAULT 'wallet',
   created_at BIGINT NOT NULL,
   UNIQUE KEY uniq_h5_usage_request_id (request_id),
   KEY idx_h5_usage_user_time (user_id, created_at),