test: them 81 pytest cho auth/orders/security/CORS/risk_manager/trade_executor/signal_service + CI

Backend truoc day chi co script goi httpx vao server dang chay that
(test_auth.py, test_full_api.py), khong phai pytest that. Them bo test
chay doc lap bang SQLite in-memory (khong can Postgres/Docker):

- test_rbac_deps.py: RBAC chain + regression-guard cho fix vai tro o /orders/place
- test_order_exchange_routing.py: routing dung san theo credential
- test_security_encryption.py: AES-GCM round-trip + tuong thich nguoc AES-CBC
- test_cors_config.py: CORS fail-closed khi thieu cau hinh
- test_risk_manager.py: Kelly sizing + SL/TP adaptive theo tung regime
- test_trade_executor.py: STRONG-only, dedup, reversal, volatility filter,
  hybrid eviction FIFO -- toan bo quy tac mo/dong trade
- test_signal_service_scoring.py: he thong cham diem 13 thuat toan

Them .gitea/workflows/backend-tests.yml chay pytest tu dong khi push/PR
dung vao backend/** (can Gitea Actions + runner da duoc bat tren instance).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/backend-tests.yml

@@ -0,0 +1,33 @@
+name: Backend tests
+
+on:
+  push:
+    branches: [master]
+    paths:
+      - "backend/**"
+      - ".gitea/workflows/backend-tests.yml"
+  pull_request:
+    paths:
+      - "backend/**"
+      - ".gitea/workflows/backend-tests.yml"
+
+jobs:
+  pytest:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt -r requirements-dev.txt
+
+      - name: Run pytest
+        run: python -m pytest tests/ -v