chore: migrate backend from pip to uv

[randroid88]

Summary

• Replace requirements.txt with pyproject.toml + uv.lock for reproducible backend dependency management
• A single uv sync command handles Python 3.11 download, venv creation, and dependency installation
• Resolve deepfilternet/numpy version conflict automatically via [tool.uv] override

Why uv?

uv is a fast Python package manager from Astral. Key benefits:

1. One command setup: cd backend && uv sync replaces the manual venv + pip install workflow
2. Reproducible builds: uv.lock pins every transitive dependency (no more "works on my machine")
3. Auto Python management: If Python 3.11 isn't installed, uv downloads it automatically
4. Resolves numpy conflict: deepfilternet declares numpy<2.0 in its metadata but works fine with numpy 2.x at runtime. whisperx requires numpy>=2.1. The [tool.uv] override-dependencies setting resolves this automatically — previously required manual --force installs

Files changed

• backend/pyproject.toml — New: project metadata + dependencies + uv override
• backend/uv.lock — New: locked dependency graph (2377 lines)
• backend/requirements.txt — Deleted (replaced by pyproject.toml)
• .gitignore — Add .venv/ (uv uses .venv by default)
• package.json — Update dev:backend script to use uv run
• README.md — Update install instructions

Test plan

• Install uv: curl -LsSf https://astral.sh/uv/install.sh | sh
• cd backend && uv sync — should create .venv/ and install all dependencies
• uv run uvicorn main:app --reload --port 8642 — backend starts successfully
• npm run dev — full dev workflow still works end-to-end