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

requirements.txt

@@ -0,0 +1,92 @@
+aiofiles==24.1.0
+    # via matrix-nio
+aiohappyeyeballs==2.6.1
+    # via aiohttp
+aiohttp==3.13.3
+    # via
+    #   aiohttp-socks
+    #   matrix-nio
+aiohttp-socks==0.11.0
+    # via matrix-nio
+aiosignal==1.4.0
+    # via aiohttp
+aiosqlite==0.22.1
+    # via matrix-paperless-ingest (pyproject.toml)
+anyio==4.12.1
+    # via httpx
+atomicwrites==1.4.1
+    # via matrix-nio
+attrs==25.4.0
+    # via
+    #   aiohttp
+    #   jsonschema
+    #   referencing
+cachetools==5.5.2
+    # via matrix-nio
+certifi==2026.2.25
+    # via
+    #   httpcore
+    #   httpx
+cffi==2.0.0
+    # via python-olm
+frozenlist==1.8.0
+    # via
+    #   aiohttp
+    #   aiosignal
+h11==0.16.0
+    # via
+    #   httpcore
+    #   matrix-nio
+h2==4.3.0
+    # via matrix-nio
+hpack==4.1.0
+    # via h2
+httpcore==1.0.9
+    # via httpx
+httpx==0.28.1
+    # via matrix-paperless-ingest (pyproject.toml)
+hyperframe==6.1.0
+    # via h2
+idna==3.11
+    # via
+    #   anyio
+    #   httpx
+    #   yarl
+jsonschema==4.26.0
+    # via matrix-nio
+jsonschema-specifications==2025.9.1
+    # via jsonschema
+matrix-nio[e2e]==0.25.2
+    # via matrix-paperless-ingest (pyproject.toml)
+multidict==6.7.1
+    # via
+    #   aiohttp
+    #   yarl
+peewee==3.19.0
+    # via matrix-nio
+propcache==0.4.1
+    # via
+    #   aiohttp
+    #   yarl
+pycparser==3.0
+    # via cffi
+pycryptodome==3.23.0
+    # via matrix-nio
+python-dotenv==1.2.2
+    # via matrix-paperless-ingest (pyproject.toml)
+python-olm==3.2.16
+    # via matrix-nio
+python-socks[asyncio]==2.8.1
+    # via aiohttp-socks
+referencing==0.37.0
+    # via
+    #   jsonschema
+    #   jsonschema-specifications
+rpds-py==0.30.0
+    # via
+    #   jsonschema
+    #   referencing
+unpaddedbase64==2.1.0
+    # via matrix-nio
+yarl==1.23.0
+    # via aiohttp