deps: Switch from uv to plain venv and pip-tools

uv installs to ~/.local/bin which is not in PATH for systemd services,
and keeping uv updated on Ubuntu requires manual steps outside of apt.
Switch to a plain Python venv with pip-tools for dependency locking.

- Replace uv venv/sync with python3 -m venv --system-site-packages
- Generate requirements.txt with pip-compile from pyproject.toml
- Update service ExecStart to use .venv/bin/python directly
- Remove uv.lock and [tool.uv] from pyproject.toml
- Update README to document venv+pip-tools workflow

matrix-paperless-ingest.service

@@ -7,7 +7,7 @@ Wants=network-online.target
 Type=simple
 WorkingDirectory=%h/matrix-paperless-ingest
 EnvironmentFile=%h/matrix-paperless-ingest/.env
-ExecStart=%h/.local/bin/uv run --no-sync python ingest.py
+ExecStart=%h/matrix-paperless-ingest/.venv/bin/python ingest.py
 Restart=on-failure
 RestartSec=10
 StandardOutput=journal