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/manifest.json

@@ -1,6 +1,6 @@
 {
-  "short_name": "SCD",
-  "name": "Steam CVE Dashboard",
+  "short_name": "AEGIS",
+  "name": "AEGIS — Advanced Engineering Group Intelligence System",
   "icons": [
     {
       "src": "favicon.ico",