test: add vitest unit tests and Gitea Actions CI workflow

Covers secrets encryption, Nixpacks runtime-version detection, the
admin-session middleware, and the installations state machine. CI
runs the same build+test steps on every push via .gitea/workflows/ci.yml.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -5,6 +5,7 @@
     "apps/*"
   ],
   "scripts": {
-    "dev:server": "npm run dev --workspace apps/server"
+    "dev:server": "npm run dev --workspace apps/server",
+    "test": "npm run test --workspace apps/server"
   }
 }