feat(billing): add billing_mode to providers (postpaid support) & UI/migration

migrations/001-add-billing-mode.sql

@@ -0,0 +1,13 @@
+-- Migration: add billing_mode to provider_configs
+-- Adds a billing_mode TEXT column with default 'prepaid'
+-- After applying, set Gemini to postpaid with:
+--   UPDATE provider_configs SET billing_mode = 'postpaid' WHERE id = 'gemini';
+
+BEGIN TRANSACTION;
+
+ALTER TABLE provider_configs ADD COLUMN billing_mode TEXT DEFAULT 'prepaid';
+
+COMMIT;
+
+-- NOTE: If you use a production SQLite file, run the following to set Gemini to postpaid:
+-- sqlite3 /path/to/db.sqlite "UPDATE provider_configs SET billing_mode='postpaid' WHERE id='gemini';"