Add PostgreSQL schema and migration runner

All tables per PRD data model with default config seeds.
Migration runner tracks applied migrations in DB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/db.js

@@ -1,7 +1,12 @@
 const { Pool } = require('pg');
+const { runMigrations } = require('../../db/migrations/001_initial_schema');
 
 const pool = new Pool({
   connectionString: process.env.DATABASE_URL,
 });
 
-module.exports = pool;
+async function initialize() {
+  await runMigrations(pool);
+}
+
+module.exports = { pool, runMigrations, initialize };