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.vision

@@ -10,7 +10,12 @@ COPY --from=ghcr.io/astral-sh/uv:0.10.9 /uv /usr/local/bin/uv
 
 # Non-root user the app runs as at runtime. Same UID as the server's Dockerfile
 # so both containers agree on ownership of the shared uploads volume (point 13).
-RUN useradd -u 10001 app
+# -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
 COPY pyproject.toml uv.lock ./