fix: ASCII-safe startup log messages + force UTF-8 stdout in the systemd unit ...

A unit on an old distro without a UTF-8 locale gets latin-1/ascii
stdout; structlog's print of the em-dash in the webclient-disabled
message raised UnicodeEncodeError and killed the app at startup
(Application startup failed, crash-loop restart counter 28). The unit
now sets PYTHONIOENCODING=utf-8 so any future non-ASCII log line is
safe too.

deploy: auto-install docker compose v2 plugin when the host lacks it ...

Ubuntu 18.04's docker packages predate compose v2. The plugin is a
single static binary that runs on any distro with docker CLI >= 18.09,
so the script downloads it into /usr/local/lib/docker/cli-plugins
(system-wide — sudo docker compose sees it too) when docker compose is
unavailable, with the same offline pattern as the python tarball: a
pre-downloaded binary next to install.sh wins. If the CLI is too old
to support plugins at all, the error asks for docker --version.

deploy: standalone CPython fallback for old distros ...

venv cannot be shipped in the repo — it embeds absolute paths and
distro-specific compiled wheels (asyncpg, uvloop, cryptography), so it
dies on any other machine. Instead, when the host has no python >= 3.11
(Ubuntu 18.04: 3.6, 22.04: 3.10; system python must not be replaced),
install.sh fetches a standalone astral python-build-standalone CPython
3.12 into .python/ — self-contained, needs only glibc >= 2.17, works
on any distro. The tarball is cached next to the repo and reused; on
offline servers a pre-downloaded tarball next to install.sh wins.
Verified locally: tarball layout, venv creation, pip install.

deploy: pick the newest python >= 3.11 instead of hardcoding python3 ...

Ubuntu 22.04's system python3 is 3.10 and must not be replaced (apt
depends on it) — a deadsnakes install lives under python3.11/3.12.
The install now tries those names before plain python3 and builds
the venv with whichever qualifies, with a hint in the error path.

deploy: regenerate placeholder secrets from the deploy branch's .env on first install

deploy: one-command server bundle (dockerized postgres + systemd) ...

deploy/install.sh — everything in one command: prerequisite checks,
.env provisioning from env.template (generated DB password), dockerized
PostgreSQL (pgvector image, restart: always, bound to 127.0.0.1, vector
+ pg_trgm extensions), venv + pip install, a systemd unit with
Restart=always (the server lives from installation, per the roadmap
where these modules become each other's subagents), health check, and
navi-code/navi-server symlinks into PATH. Idempotent — safe to re-run.

Default deployed shape: web UI off, auth off, terminal client only.
Re-enabling the web panel later is editing .env + restart — nothing is
removed from the tree.

The deploy branch (next commit set) differs from master only by a
filled-in .env.