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>

apps/server/package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "dev": "tsx src/index.ts",
     "build": "tsc -p tsconfig.json",
-    "start": "node dist/index.js"
+    "start": "node dist/index.js",
+    "test": "vitest run"
   },
   "dependencies": {
     "better-sqlite3": "11.10.0",
@@ -22,6 +23,7 @@
     "@types/http-proxy": "1.17.16",
     "@types/node": "24.9.1",
     "tsx": "4.20.6",
-    "typescript": "5.9.3"
+    "typescript": "5.9.3",
+    "vitest": "^5.0.3"
   }
 }