Add OAuth2/OIDC per-user Gitea authentication

Introduce a GiteaOAuthValidator for JWT and userinfo validation and
fallbacks, add /oauth/token proxy, and thread per-user tokens through
the
request context and automation paths. Update config and .env.example for
OAuth-first mode, add OpenAPI, extensive unit/integration tests,
GitHub/Gitea CI workflows, docs, and lint/test enforcement (>=80% cov).

.gitea/workflows/lint.yml

@@ -0,0 +1,34 @@
+name: lint
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements-dev.txt
+
+      - name: Run Ruff (fail on any diagnostics)
+        run: ruff check src tests
+
+      - name: Enforce formatting
+        run: |
+          ruff format --check src tests
+          black --check src tests
+
+      - name: Run mypy
+        run: mypy src