fix(MAT-258): blacklist unverified E2EE devices + add CI tests

Unverified devices (lacking cross-signing) caused OlmUnverifiedDeviceError
in _send_text(), silently breaking all message delivery. Now on_sync()
blacklists non-cross-signed devices instead of skipping them, and
_send_text() catches E2EE errors gracefully.

Adds 12 unit tests for device trust policy and send error handling.
CI test job now gates deployment in deploy.yml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -0,0 +1,21 @@
+name: Tests
+on:
+  push:
+    branches: [main]
+    paths-ignore: ['**.md', 'docs/**']
+  pull_request:
+    branches: [main]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+      - name: Install dependencies
+        run: |
+          pip install -r requirements.txt -r requirements-test.txt
+      - name: Run tests
+        run: |
+          pytest tests/ -v --cov=device_trust --cov-report=term