Rebrand STEAM → AEGIS, fix BU drift checker previous_bu bug

- Replace all STEAM branding with AEGIS (Advanced Engineering Group
  Intelligence System) across login, header, nav drawer, manifest, and
  browser title
- Add shield logo to login page, main header, and nav drawer
- Fix BU drift checker recording incorrect previous_bu values by
  building a previousBuMap snapshot BEFORE the upsert/delete cycle
  instead of querying the DB after rows are already gone
- Clean 526 bogus BU history entries generated by the broken logic
- Add docs and scripts from prior session

frontend/public/index.html

@@ -7,7 +7,7 @@
     <meta name="theme-color" content="#000000" />
     <meta
       name="description"
-      content="Steam CVE Dashboard - Vulnerability tracking and documentation"
+      content="AEGIS — Advanced Engineering Group Intelligence System"
     />
     <link rel="apple-touch-icon" href="%PUBLIC_URL%/logo192.png" />
     <!--
@@ -24,7 +24,7 @@
       work correctly both with client-side routing and a non-root public URL.
       Learn how to configure a non-root public URL by running `npm run build`.
     -->
-    <title>CVE Dashboard</title>
+    <title>AEGIS</title>
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
     <link href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;500;600;700&family=Outfit:wght@300;400;500;600;700&display=swap" rel="stylesheet">