refactor: tech debt reduction - extract hooks, deduplicate helpers, update deps, add CI

- Extract useCustomStepFlow hook from TreeNavigationPage (1040 → 759 lines)
- Create core/filters.py with shared tree/step visibility filters
- Create services/export_service.py from session export logic
- Add GitHub Actions CI/CD pipeline (pytest + lint + build)
- Add GIN index migration for full-text search on trees
- Update FastAPI 0.128.5, Pydantic 2.12.5, SQLAlchemy 2.0.46, +5 more
- Fix regex → pattern deprecation in Query() params

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,73 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  backend:
+    runs-on: ubuntu-latest
+
+    services:
+      postgres:
+        image: postgres:16
+        env:
+          POSTGRES_USER: postgres
+          POSTGRES_PASSWORD: postgres
+          POSTGRES_DB: patherly_test
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd pg_isready
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+
+    env:
+      DATABASE_URL: postgresql+asyncpg://postgres:postgres@localhost:5432/patherly_test
+      DATABASE_URL_SYNC: postgresql://postgres:postgres@localhost:5432/patherly_test
+      SECRET_KEY: ci-test-secret-key-not-for-production
+      DEBUG: "true"
+      APP_NAME: ResolutionFlow
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python 3.11
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+          cache: pip
+          cache-dependency-path: |
+            backend/requirements.txt
+            backend/requirements-dev.txt
+
+      - name: Install dependencies
+        run: pip install -r backend/requirements.txt -r backend/requirements-dev.txt
+
+      - name: Run tests
+        run: cd backend && python -m pytest --override-ini="addopts="
+
+  frontend:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Node.js 20
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: npm
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install dependencies
+        run: cd frontend && npm ci
+
+      - name: Lint
+        run: cd frontend && npm run lint
+
+      - name: Build
+        run: cd frontend && npm run build