MySQL data layer: accounts, sessions, campaigns, burns (Marty: real concurrency)

Coolify MySQL (instantadpay-db) via DATABASE_URL; db.js bootstraps schema
and one-time imports the volume JSON. accounts/auth/ads are dual-mode: the
MySQL path uses guarded UPDATEs for the concurrent ad-serving hot path;
without DATABASE_URL the JSON stores remain (local dev). All data functions
async; server boots through db.init. Chain index stays a file: it is a
rebuildable cache of the blockchain, which remains the money truth.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -5,5 +5,6 @@
   "description": "InstantAdPay membership advertising site - immutable on-chain settlement, transparent ledger.",
   "main": "server.js",
   "scripts": {"start": "node server.js", "dev": "node --watch server.js"},
-  "engines": {"node": ">=20"}
+  "engines": {"node": ">=20"},
+  "dependencies": {"mysql2": "^3.11.0"}
 }