add dora-webcam node

Captures frames from a V4L2 camera and publishes them as std/media/v1/Image:
an Arrow struct of width, height, encoding and data, so a receiver needs
nothing from the message metadata. For a compressed encoding the recorded
dimensions are the image's, not the length of the payload.

Based on opencv-video-capture from dora-hub, with three changes. The output is
the typed image struct rather than a flat UInt8 buffer plus metadata. Camera
selection by id reads /dev/v4l/by-id/ only; the upstream macOS system_profiler
and Windows PowerShell enumeration is not carried. CAPTURE_FRAMES bounds a run
so a test ends on its own, in place of upstream's ten-second CI cutoff -- a
timeout would have made the CI step exit non-zero.

CI=true keeps a failed capture from raising, publishing a labelled black frame
instead, which is what lets dataflow.yml run on a machine with no camera.

Two dataflows, run one at a time, as dora has no cross-dataflow wiring:
dataflow.yml is the camera alone and the CI gate; dataflow_view.yml wires it
into dora-image-view and is not run by CI, since it fetches that node over the
network and needs a tag to exist there.

.gitea/workflows/validate.yml

@@ -0,0 +1,86 @@
+# Gates that need no camera: the dataflow still matches the typed contract in
+# dora-node.yml and still runs end to end, the unit tests pass, and the docs
+# lint. CI=true is set by the runner, which is what makes a failed capture
+# publish a placeholder frame instead of raising.
+name: validate
+
+# A pull request would otherwise run this twice on the same commit: once for
+# the branch push and once for the PR.
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+env:
+  DORA_VERSION: v1.0.1
+  DORA_TARGET: x86_64-unknown-linux-gnu
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      # The opencv-python wheel links against libGL at import time, so cv2
+      # cannot even be imported on the bare runner image.
+      - name: Install the OpenCV shared libraries
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends libgl1 libglib2.0-0
+
+      - uses: astral-sh/setup-uv@v5
+        with:
+          enable-cache: true
+
+      # The CLI is a single static binary pinned to DORA_VERSION, so the cache
+      # hit is exact and the download only happens when the version changes.
+      - name: Cache the dora CLI
+        id: dora-cache
+        uses: actions/cache@v4
+        with:
+          path: ~/.local/bin/dora
+          key: dora-cli-${{ env.DORA_VERSION }}-${{ env.DORA_TARGET }}
+
+      - name: Install the dora CLI
+        if: steps.dora-cache.outputs.cache-hit != 'true'
+        run: |
+          curl -fsSL -o dora-cli.tar.gz \
+            "https://github.com/dora-rs/dora/releases/download/${DORA_VERSION}/dora-cli-${DORA_TARGET}.tar.gz"
+          tar xzf dora-cli.tar.gz
+          mkdir -p "$HOME/.local/bin"
+          install -m 755 "dora-cli-${DORA_TARGET}/dora" "$HOME/.local/bin/dora"
+          rm -rf dora-cli.tar.gz "dora-cli-${DORA_TARGET}"
+
+      - name: Put the dora CLI on PATH
+        run: echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+
+      - run: dora --version
+
+      # --locked fails if uv.lock is out of date with pyproject.toml, so a
+      # dependency change that was never locked cannot reach main.
+      - name: Install
+        run: uv sync --locked --group dev
+
+      - run: uv run ruff check .
+
+      - run: uv run pymarkdown scan *.md
+
+      - run: uv run pytest -q
+
+      - name: Validate the node manifest
+        run: dora validate --node-manifest dora-node.yml
+
+      # strict_types: true in the dataflow turns a type warning into a non-zero
+      # exit, so a mismatched annotation fails here rather than at runtime.
+      - name: Validate the dataflow
+        run: dora validate dataflow.yml
+
+      # dataflow_view.yml is deliberately not run: it fetches dora-image-view
+      # over the network and needs a tag to exist there.
+      # CAPTURE_FRAMES bounds the run so it ends on its own; CI=true is already
+      # set by the runner, which makes a failed capture publish a placeholder
+      # frame rather than raising on a machine with no camera.
+      - name: Run the dataflow headless
+        env:
+          CAPTURE_FRAMES: 30
+        run: dora run dataflow.yml --uv