v2.0: dual deployment — pip install (SQLite) + Docker (PostgreSQL)

Architecture:
- Moved backend/app/ to diligence/ Python package
- Dialect-agnostic models (Uuid + JSON, no PostgreSQL imports)
- Auto-detect database: empty DATABASE_URL → SQLite at ~/.diligence/data.db
- Cross-dialect migrations (inspector-based, no raw PostgreSQL DDL)
- FastAPI serves pre-built React frontend for pip path
- CLI entry point: diligence [--port] [--no-browser] [--data-dir]
- pyproject.toml with optional deps: [postgres], [mcp], [dev]
- Docker path unchanged: docker compose up -d

pip install path: pip install . && diligence
Docker path: ./setup.sh && docker compose up -d

backend/Dockerfile

@@ -1,7 +1,12 @@
 FROM python:3.12-slim
+
 WORKDIR /app
+
 RUN apt-get update && apt-get install -y --no-install-recommends curl && rm -rf /var/lib/apt/lists/*
-COPY requirements.txt .
+
+COPY backend/requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
-COPY . .
-CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]
+
+COPY diligence/ ./diligence/
+
+CMD ["uvicorn", "diligence.main:app", "--host", "0.0.0.0", "--port", "8000"]