feat(6): add Caddy reverse proxy with SSL to standalone deployment

Complete feature parity with Docker deployment:
  - Caddy reverse proxy manages SSL/TLS
  - Ports: 8916/8917 (HTTP), 8918/8919 (HTTPS)
  - Self-signed certificates (development-friendly)
  - Automatic certificate generation via on-demand TLS
  - Security headers (HSTS, XSS Protection, etc.)

Changes:
  - Created Caddyfile.standalone for localhost config
  - Enhanced start_servers.py with Caddy startup/monitoring
  - Auto-install Caddy via apt if not present
  - Updated status output to show both HTTP and HTTPS URLs
  - All three services (backend, frontend, caddy) in one launcher

Both Docker and Standalone modes now have IDENTICAL capabilities:
  - Full production-ready SSL/TLS support
  - Reverse proxy with auto-certificate management
  - HTTPS-only capable

Resolves dual-deployment equivalence requirement

Caddyfile.standalone

@@ -0,0 +1,40 @@
+# TFM aInventory - Standalone Caddy Configuration
+# Self-signed SSL/TLS reverse proxy for localhost deployment
+{
+    admin off
+    local_certs
+    skip_install_trust
+
+    on_demand_tls {
+        ask http://localhost:8916/
+    }
+}
+
+# HTTPS Frontend (port 8919 -> 443)
+https://localhost:8919 {
+    tls internal {
+        on_demand
+    }
+    reverse_proxy localhost:8917
+
+    header {
+        Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
+        X-XSS-Protection "1; mode=block"
+        X-Content-Type-Options "nosniff"
+        X-Frame-Options "SAMEORIGIN"
+        Referrer-Policy "strict-origin-when-cross-origin"
+    }
+}
+
+# HTTPS Backend (port 8918 -> 444)
+https://localhost:8918 {
+    tls internal {
+        on_demand
+    }
+    reverse_proxy localhost:8916
+
+    header {
+        Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
+    }
+}
+}