Migrate from JSON to PostgreSQL, update SLA to 99.5%

- Replace JSON file storage with PostgreSQL (pg package)
- Add db.ts service for connection pool and schema init
- Rewrite keys.ts, verification.ts, usage.ts for async PostgreSQL
- Update all routes for async function signatures
- Add migration script (scripts/migrate-to-postgres.mjs)
- Update docker-compose.yml with DATABASE_* env vars
- Change SLA from 99.9% to 99.5% in landing page

public/index.html

@@ -256,7 +256,7 @@ html, body {
         <div class="trust-label">Avg. generation time</div>
       </div>
       <div class="trust-item">
-        <div class="trust-num">99.9%</div>
+        <div class="trust-num">99.5%</div>
         <div class="trust-label">Uptime SLA</div>
       </div>
       <div class="trust-item">