feat(01-01): Backend FastAPI skeleton + Docker build config

- FastAPI app with SPAStaticFiles catch-all (root_path NOT in constructor)
- config.py with DATA_PATH, PROMPTS_PATH, OUTPUTS_PATH, CAMPAIGNS_PATH, CONFIG_PATH
- Startup lifespan creates data directories automatically
- Health endpoint: GET /api/health -> {"status": "ok"}
- Dockerfile multi-stage: node:22-slim builds React, python:3.12-slim serves API+SPA
- --root-path /postgenerator set in Uvicorn CMD only (avoids Pitfall #4)
- docker-compose.yml: lab-postgenerator-app, proxy_net, named volume for data persistence
- requirements.txt with pinned versions: fastapi[standard]==0.135.1, anthropic==0.84.0

Dockerfile

@@ -0,0 +1,41 @@
+# ============================================================
+# Stage 1: Build React SPA
+# ============================================================
+FROM node:22-slim AS frontend-builder
+
+WORKDIR /app/frontend
+
+# Install dependencies first (layer cache optimization)
+COPY frontend/package*.json ./
+RUN npm ci
+
+# Copy source and build
+COPY frontend/ ./
+RUN npm run build
+
+# ============================================================
+# Stage 2: Python runtime (serves API + built SPA)
+# ============================================================
+FROM python:3.12-slim AS runtime
+
+WORKDIR /app
+
+# Install Python dependencies
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy backend source
+COPY backend/ ./backend/
+
+# Copy built React SPA from frontend-builder stage
+COPY --from=frontend-builder /app/frontend/dist ./static
+
+# Create data directory structure
+RUN mkdir -p /app/data/prompts /app/data/outputs /app/data/campaigns /app/data/config
+
+# Expose port
+EXPOSE 8000
+
+# CRITICAL: --root-path /postgenerator is set HERE (Uvicorn), NOT in FastAPI() constructor
+# This avoids the double-path bug (Pitfall #4)
+CMD ["uvicorn", "backend.main:app", "--host", "0.0.0.0", "--port", "8000", "--root-path", "/postgenerator"]