Migrate dependency management to uv

Replace requirements.txt with pyproject.toml (direct deps only; uv
resolves the rest into uv.lock). Point the systemd unit at the
uv-managed .venv, and gitignore it.

Run `uv lock` to generate the lockfile and commit it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyproject.toml

@@ -0,0 +1,16 @@
+[project]
+name = "food"
+version = "0.1.0"
+description = "Pantry + meta-recipe meal planning (Django, HTMX)"
+requires-python = ">=3.11"
+dependencies = [
+    "django==5.2.12",
+    "djangorestframework==3.17.1",
+    "whitenoise==6.12.0",
+    "gunicorn==25.3.0",
+    "recipe-scrapers==15.11.0",
+    "requests==2.33.1",
+]
+
+# No [build-system]: this is an application, not an installable package, so uv
+# treats it as a virtual project and won't try to build/install it.