feat: comprehensive security, performance, UX and deployment audit

Security:
- CORS restricted to explicit methods/headers, configurable via CORS_ORIGINS env
- Auth added to /reports/{rid}/html and /compliance-report endpoints
- Ownership check on report downloads
- Rate limiting on login (10 attempts/5min per IP with threading.Lock)
- Non-root container user (agent via gosu entrypoint)
- Nginx security headers (X-Frame-Options, X-Content-Type-Options, X-XSS-Protection, Referrer-Policy, Permissions-Policy)
- .gitignore and .dockerignore for secret leak prevention
- .env.example with documentation

Performance:
- 16 SQLite indexes on foreign keys and frequently queried columns
- Pagination on chat messages (100), reports (50), audit log (100)
- subprocess.run wrapped in run_in_executor (3 async handlers)
- asyncio.wait_for timeouts on GenAI calls (300s) and Chromium PDF (120s)
- Thread pool reduced to 10 workers
- Code splitting with React.lazy (bundle: 1.3MB -> ~550KB initial)
- React.memo on TreeItem (recursive compartment tree)

Error handling:
- 13 bare except clauses replaced with logged Exception handlers
- Graceful shutdown handler (terminates subprocesses + executor)
- File upload validation (50MB max, extension whitelist per endpoint)
- Health check expanded (version, uptime, db_ok)
- Cache-Control on /api/genai/models (1h)
- Auto-cleanup audit_log > 30 days

Dead code removed:
- DownloadsPage.tsx, StubPage.tsx, MfaPage.tsx (moved to UsersPage)
- Legacy frontend/ directory
- 19 unused i18n keys, dist/ removed from git tracking

UX & i18n:
- 8 alert() calls replaced with styled error states (TerraformPage)
- 50+ hardcoded Portuguese strings localized to i18n (pt/en) across 11 files
- aria-label on all icon-only buttons (Chat, Explorer, Sidebar)
- focus-visible CSS for keyboard navigation
- Responsive grid fix (360px -> 280px for mobile)
- aria-hidden on decorative SVGs

Deployment:
- Docker resource limits (backend 4G, frontend 512M)
- Log rotation (json-file, 10MB x 3)
- Terraform version parameterized via ARG

nginx/default.conf

@@ -1,3 +1,7 @@
+map $uri $empty {
+    default "";
+}
+
 server {
     listen 80;
     server_name _;
@@ -9,7 +13,12 @@ server {
     location / {
         root /usr/share/nginx/html/app;
         try_files $uri $uri/ /index.html;
-        add_header Cache-Control "no-cache, no-store, must-revalidate";
+        add_header Cache-Control "no-cache, no-store, must-revalidate" always;
+        add_header X-Frame-Options "DENY" always;
+        add_header X-Content-Type-Options "nosniff" always;
+        add_header X-XSS-Protection "1; mode=block" always;
+        add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+        add_header Permissions-Policy "geolocation=(), microphone=(), camera=()" always;
     }
 
     # Backend API proxy
@@ -22,5 +31,7 @@ server {
         proxy_read_timeout 900s;
         proxy_send_timeout 900s;
         proxy_connect_timeout 60s;
+        add_header X-Frame-Options "DENY" always;
+        add_header X-Content-Type-Options "nosniff" always;
     }
 }