ci: add Gitea Actions build + test pipeline

Real CI at .gitea/workflows/ci.yml (Gitea reads .gitea/workflows, not the empty
untracked .git/workflows/ci.yml cruft): backend dotnet build + dotnet test (39 tests)
and frontend npm ci + build, on pushes to main/develop and all PRs. Establishes the
merge gate recommended in the Phase 7 git-workflow review.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: CI
+
+# Build + test gate. Runs on pushes to the long-lived branches and on every PR so
+# the 39-test suite (and both builds) must pass before merge.
+on:
+  push:
+    branches: [main, develop]
+  pull_request:
+
+jobs:
+  backend:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: '8.0.x'
+      - name: Restore
+        run: dotnet restore InboxIntel.sln
+      - name: Build
+        run: dotnet build InboxIntel.sln -c Release --no-restore
+      - name: Test
+        run: dotnet test InboxIntel.sln -c Release --no-build --verbosity normal
+
+  frontend:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: frontend
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+      - name: Install
+        run: npm ci
+      - name: Build
+        run: npm run build