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/src/App.js

@@ -1022,10 +1022,15 @@ export default function App() {
                 <Menu className="w-5 h-5" />
               </button>
               <div>
-                <h1 className="text-4xl font-bold text-intel-accent mb-1 font-mono tracking-tight">
-                  STEAM Security Dashboard
-                </h1>
-                <p className="text-gray-400 text-sm font-sans">NTS Threat Intelligence and Metric Aggregation</p>
+                <div className="flex items-center gap-3">
+                  <img src="/shieldlogo.jpeg" alt="AEGIS" style={{ width: '44px', height: '44px', borderRadius: '6px' }} />
+                  <div>
+                    <h1 className="text-4xl font-bold text-intel-accent mb-1 font-mono tracking-tight">
+                      AEGIS
+                    </h1>
+                    <p className="text-gray-400 text-sm font-sans">Advanced Engineering Group Intelligence System</p>
+                  </div>
+                </div>
               </div>
             </div>
             <div className="flex items-center gap-3">