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.