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/requirements.txt

@@ -1,12 +1,14 @@
-# Backend
+# Core
 fastapi==0.115.6
 uvicorn[standard]==0.34.0
 sqlalchemy[asyncio]==2.0.36
-asyncpg==0.30.0
-alembic==1.14.0
 pydantic==2.10.3
 pydantic-settings==2.7.0
 python-jose[cryptography]==3.3.0
 bcrypt==4.2.1
 httpx==0.28.1
 python-multipart==0.0.18
+
+# Database drivers
+asyncpg==0.30.0
+aiosqlite==0.20.0