feat: add dual Tailscale/Headscale serve support + template improvements

- Add serve.json (Tailscale with certs) and serve.headscale.json (Headscale/HTTP-only)
- Pin Vikunja to v0.24
- Improve .env.example with comments
- Harden .gitignore (env, keys, Tailscale state)
- Add SECURITY.md
- Update README with configuration and usage instructions
- Remove all damconsulting references for generic template use

tailscale/config/serve.json

@@ -1,19 +1,16 @@
 {
-    "TCP": {
-      "443": {
-        "HTTPS": true
-      }
-    },
-    "Web": {
-      "${TS_CERT_DOMAIN}:443": {
-        "Handlers": {
-          "/": {
-            "Proxy": "http://127.0.0.1:3000"
-          }
+  "TCP": {
+    "80": {
+      "HTTP": true
+    }
+  },
+  "Web": {
+    "vikunja.example.com:80": {
+      "Handlers": {
+        "/": {
+          "Proxy": "http://127.0.0.1:3456"
         }
       }
-    },
-    "AllowFunnel": {
-      "${TS_CERT_DOMAIN}:443": false
     }
-  }
+  }
+}