fix(nexus-db): enforce UTF8 encoding at install time (issue #8)

A PostgreSQL cluster/database freezes its encoding at initdb / CREATE
DATABASE time; a C (non-UTF-8) locale yields a SQL_ASCII cluster, which
makes psycopg3 return bytes and crashes SQLAlchemy. Harden the installer
and add a reusable pattern for future DB installers:

- ensure_utf8_locale_active: generate AND activate en_US.UTF-8 for the
  install process before the server package runs initdb; abort if the
  locale is not actually available
- create the database explicitly with TEMPLATE template0 ENCODING 'UTF8'
  LC_COLLATE/LC_CTYPE 'en_US.UTF-8' instead of inheriting the cluster
  default
- assert_db_encoding_utf8: post-install guard, abort with an actionable
  message if pg_encoding_to_char is not UTF8 (catches old SQL_ASCII DBs
  on re-run too)
- credentials/README docs use su - postgres -c (minimal LXCs have no sudo)

README.md

@@ -41,6 +41,8 @@ Two files per app, both sourcing the shared libs via `curl`:
 - **`ct/<app>.sh`** runs on the PVE host: prompts → unprivileged LXC → pushes a config env file into the container → bootstraps the installer. Apps that need Docker (authentik, runner) enable `nesting+keyctl` automatically. The standard prompts include an **SSH root login choice** (`SSH_ROOT_LOGIN`, default yes for homelab convenience; `no` keeps the Debian key-only default) — applied inside the container as an sshd drop-in by `configure_ssh_root_login`.
 - **`install/<app>-install.sh`** runs inside the LXC: packages, unprivileged app user, secrets generated on-host (never printed), systemd units, a `/root/<app>.credentials` notes file — then shreds the bootstrap env. Idempotent where it matters: re-runs skip what exists. `setup_base_apt` also fixes the bare-template **locale situation**: `C.UTF-8` is exported up front (glibc built-in, covers the first apt run without perl warnings), then `en_US.UTF-8` is generated and set as the system default.
 
+**DB installers** carry one extra rule: a PostgreSQL cluster/database freezes its encoding at initdb / `CREATE DATABASE` time and it can never be changed afterwards. A C (non-UTF-8) locale yields a `SQL_ASCII` cluster — psycopg3 then hands text back as bytes and SQLAlchemy crashes. So the pattern (helpers `ensure_utf8_locale_active` + `assert_db_encoding_utf8` in `lib/install.func`) is: make a UTF-8 locale **active** before the server package runs initdb, create the database **explicitly** with `TEMPLATE template0 ENCODING 'UTF8' LC_COLLATE/LC_CTYPE 'en_US.UTF-8'` (never inherit the cluster default), and **verify** `pg_encoding_to_char` returns `UTF8` before finishing — a wrong encoding aborts the install (it's DB damage, see wiki → Lessons). Maintenance examples use `su - postgres -c …`, not `sudo` — these minimal LXCs have no sudo.
+
 Shared libs: [`lib/build.func`](lib/build.func) (host-side: prompts, LXC create, bootstrap) and [`lib/install.func`](lib/install.func) (in-container: apt, users, systemd, http-wait).
 
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