fix(docker): home dir e cache uv per l'utente non privilegiato (#38.1)

useradd senza -m lascia l'utente app senza $HOME: uv run nel CMD non
trova dove scrivere la sua cache. Aggiunto -m a useradd in Dockerfile,
Dockerfile.frontend e Dockerfile.vision, e UV_CACHE_DIR=/tmp/uv-cache
esplicito (indipendente da $HOME). docs/DEPLOYMENT.md: nuova sezione
sull'aggiornamento di un'installazione esistente, con il chown del
volume upload_data ereditato da versioni precedenti.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile.frontend

@@ -18,8 +18,13 @@ FROM python:3.11.16-slim AS base
 # uv from the official slim image (fast Python package manager).
 COPY --from=ghcr.io/astral-sh/uv:0.10.9 /uv /uvx /usr/local/bin/
 
-# Non-root user the app runs as at runtime.
-RUN useradd -u 10001 app
+# Non-root user the app runs as at runtime. -m creates a home dir: `uv run`
+# in the CMD below writes its cache under $HOME, and without one it has
+# nowhere writable to put it.
+RUN useradd -m -u 10001 app
+# Independent of $HOME regardless: keeps the cache path explicit and writable
+# by the app user even if the home directory assumption ever changes.
+ENV UV_CACHE_DIR=/tmp/uv-cache
 
 WORKDIR /app