Add proper login to Admin UI, replace Caddy basic_auth

- iron-session for encrypted httpOnly session cookies
- POST /api/auth/login: verifies ADMIN_USERNAME/ADMIN_PASSWORD, sets session, returns API key
- POST /api/auth/logout: destroys session
- middleware.ts: protects all routes, redirects to /login if unauthenticated
- Login page: username + password form (no more browser popup)
- Sidebar: logout calls API route and clears session
- docker-compose.prod.yml: admin-ui reads /opt/rustycms/.env.admin
- deploy.yml: generates .env.admin from Gitea secrets
- Caddy: basic_auth removed from /admin* block

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

admin-ui/package-lock.json

@@ -15,6 +15,7 @@
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
         "cmdk": "^1.1.1",
+        "iron-session": "^8.0.4",
         "lucide-react": "^0.577.0",
         "next": "16.1.6",
         "next-intl": "^4.8.3",
@@ -5958,6 +5959,15 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/cookie": {
+      "version": "0.7.2",
+      "resolved": "https://registry.npmjs.org/cookie/-/cookie-0.7.2.tgz",
+      "integrity": "sha512-yki5XnKuf750l50uGTllt6kKILY4nQ1eNIQatoXEByZ5dWgnKqbnqmTrBE5B4N7lrMJKQ2ytWMiTO2o0v6Ew/w==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 0.6"
+      }
+    },
     "node_modules/cross-spawn": {
       "version": "7.0.6",
       "resolved": "https://registry.npmjs.org/cross-spawn/-/cross-spawn-7.0.6.tgz",
@@ -7699,6 +7709,30 @@
         "tslib": "^2.8.1"
       }
     },
+    "node_modules/iron-session": {
+      "version": "8.0.4",
+      "resolved": "https://registry.npmjs.org/iron-session/-/iron-session-8.0.4.tgz",
+      "integrity": "sha512-9ivNnaKOd08osD0lJ3i6If23GFS2LsxyMU8Gf/uBUEgm8/8CC1hrrCHFDpMo3IFbpBgwoo/eairRsaD3c5itxA==",
+      "funding": [
+        "https://github.com/sponsors/vvo",
+        "https://github.com/sponsors/brc-dd"
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "cookie": "^0.7.2",
+        "iron-webcrypto": "^1.2.1",
+        "uncrypto": "^0.1.3"
+      }
+    },
+    "node_modules/iron-webcrypto": {
+      "version": "1.2.1",
+      "resolved": "https://registry.npmjs.org/iron-webcrypto/-/iron-webcrypto-1.2.1.tgz",
+      "integrity": "sha512-feOM6FaSr6rEABp/eDfVseKyTMDt+KGpeB35SkVn9Tyn0CqvVsY3EwI0v5i8nMHyJnzCIQf7nsy3p41TPkJZhg==",
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/brc-dd"
+      }
+    },
     "node_modules/is-alphabetical": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/is-alphabetical/-/is-alphabetical-2.0.1.tgz",
@@ -11281,6 +11315,12 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/uncrypto": {
+      "version": "0.1.3",
+      "resolved": "https://registry.npmjs.org/uncrypto/-/uncrypto-0.1.3.tgz",
+      "integrity": "sha512-Ql87qFHB3s/De2ClA9e0gsnS6zXG27SkTiSJwjCc9MebbfapQfuPzumMIUMi38ezPZVNFcHI9sUIepeQfw8J8Q==",
+      "license": "MIT"
+    },
     "node_modules/undici-types": {
       "version": "6.21.0",
       "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-6.21.0.tgz",