ci(coverage-flake-probe): add workflow_dispatch matrix job (20 parallel runs)

Verification mechanism for the 20-run acceptance criterion of issue #553.
Triggered manually via workflow_dispatch, runs the full coverage suite 20×
in parallel against a single SHA, asserts zero `[birpc] rpc is closed`
lines in every cell.

One fire, parallel cost (~one main-job's wall-clock), deterministic signal
for the teardown race. Cheaper than 20 sequential push events and tests
the same property the AC names.

Closes the verification gap raised by Tobias and Elicit in the issue
discussion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/coverage-flake-probe.yml

@@ -0,0 +1,64 @@
+name: Coverage Flake Probe
+
+# Manually-triggered probe for the birpc teardown race documented in ADR 012
+# / #553. Runs the full coverage suite 20× in parallel against a single SHA
+# and asserts zero `[birpc] rpc is closed` lines across every cell. Verifies
+# the acceptance criterion that the race no longer surfaces under coverage.
+
+on:
+  workflow_dispatch:
+
+jobs:
+  coverage-flake-probe:
+    name: Coverage flake probe (run ${{ matrix.run }})
+    runs-on: ubuntu-latest
+    container:
+      image: mcr.microsoft.com/playwright:v1.58.2-noble
+    strategy:
+      fail-fast: false
+      matrix:
+        run: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Cache node_modules
+        id: node-modules-cache
+        uses: actions/cache@v4
+        with:
+          path: frontend/node_modules
+          key: node-modules-${{ hashFiles('frontend/package-lock.json') }}
+
+      - name: Install dependencies
+        if: steps.node-modules-cache.outputs.cache-hit != 'true'
+        run: npm ci
+        working-directory: frontend
+
+      - name: Compile Paraglide i18n
+        run: npx @inlang/paraglide-js compile --project ./project.inlang --outdir ./src/lib/paraglide
+        working-directory: frontend
+
+      - name: Run unit and component tests with coverage
+        shell: bash
+        run: |
+          set -eo pipefail
+          npm run test:coverage 2>&1 | tee /tmp/coverage-test-${{ github.run_id }}-${{ matrix.run }}.log
+        working-directory: frontend
+        env:
+          TZ: Europe/Berlin
+
+      - name: Assert no birpc teardown race
+        shell: bash
+        if: always()
+        run: |
+          if grep -qF "[birpc] rpc is closed" /tmp/coverage-test-${{ github.run_id }}-${{ matrix.run }}.log 2>/dev/null; then
+            echo "FAIL: [birpc] rpc is closed teardown race detected in run ${{ matrix.run }}"
+            grep -F "[birpc] rpc is closed" /tmp/coverage-test-${{ github.run_id }}-${{ matrix.run }}.log
+            exit 1
+          fi
+
+      - name: Upload coverage log on failure
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-log-run-${{ matrix.run }}
+          path: /tmp/coverage-test-${{ github.run_id }}-${{ matrix.run }}.log