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>

.gitea/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+      - run: npm ci
+      - run: npm run build --workspace apps/server
+      - run: npm test