ci: add GitHub Actions workflow and ESLint v9 config

Adds two-job CI (backend + frontend) running ruff, pytest (unit only -
skipping heavy ML deps), docker compose config validation for both dev
and prod overlays, plus npm ci -> eslint -> tsc -> vite build for the
frontend.

ESLint config uses the v9 flat-config format that the project was
already on (eslint v9 dropped .eslintrc support); replaces the broken
'eslint . --ext' invocation and adds @typescript-eslint, react-hooks,
and react-refresh plugins.

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

.github/workflows/ci.yml

@@ -0,0 +1,86 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  backend:
+    name: Backend (lint + tests + compose)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+          cache: "pip"
+
+      - name: Install (light deps only, skip ocrmypdf/docling/torch)
+        run: |
+          python -m pip install --upgrade pip
+          pip install \
+            "pydantic>=2.7" "pydantic-settings>=2.4" \
+            "sqlalchemy>=2.0.30" "psycopg[binary]>=3.2" \
+            "alembic>=1.13" "minio>=7.2.7" \
+            "opensearch-py>=2.6" "qdrant-client>=1.10" \
+            "celery>=5.4" "redis>=5.0" \
+            "fastapi>=0.115" "uvicorn[standard]>=0.30" \
+            "structlog>=24.2" "tenacity>=8.5" "langdetect>=1.0.9" \
+            "regex>=2024.5.15" "click>=8.1.7" \
+            "pytest>=8.2" "pytest-asyncio>=0.23" \
+            ruff
+
+      - name: Ruff
+        working-directory: legacy-knowledge-indexer
+        run: ruff check app scripts tests
+
+      - name: Compile
+        working-directory: legacy-knowledge-indexer
+        run: python -m compileall -q app scripts tests
+
+      - name: Pytest (unit only — heavy deps excluded)
+        working-directory: legacy-knowledge-indexer
+        run: |
+          pytest tests/test_hashing.py tests/test_quality.py \
+                 tests/test_chunker.py tests/test_duplicates.py \
+                 tests/test_hybrid_search.py -q
+
+      - name: docker compose config
+        working-directory: legacy-knowledge-indexer
+        run: docker compose config --quiet
+
+      - name: docker compose config (prod overlay)
+        working-directory: legacy-knowledge-indexer
+        run: |
+          test -f docker-compose.prod.yml && \
+            docker compose -f docker-compose.yml -f docker-compose.prod.yml config --quiet || \
+            echo "prod overlay absent, skipping"
+
+  frontend:
+    name: Frontend (lint + type-check + build)
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: legacy-knowledge-indexer/frontend
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: "npm"
+          cache-dependency-path: legacy-knowledge-indexer/frontend/package-lock.json
+
+      - run: npm ci
+
+      - name: ESLint
+        run: npm run lint
+
+      - name: TypeScript
+        run: npx tsc --noEmit
+
+      - name: Build
+        run: npx vite build