feat(deb): package the server, and ask the questions that fail silently

DR-015, DR-016. §8 already shipped a static binary and an optional
container; this adds the third form, and it packages the SAME binary the
musl job proved static rather than building its own. Two builds of the
same commit could diverge, and the whole point of that assertion is that
the artifact an operator installs is the one that was checked.

Built with dpkg-deb from an explicit staging tree rather than cargo-deb.
debconf's `config` script and `templates` live in the control archive
next to the maintainer scripts, and controlling that archive directly
beats discovering what a wrapper will copy into it. dpkg-dev is on every
Debian builder, so this adds no build dependency.

Why debconf at all: two settings fail SILENTLY when unset. Without
JRAY_TMDB_API_KEY every upload stays `pending` and is never listed;
without JRAY_TRUSTED_PROXIES the X-Forwarded-For header is ignored, so
every client shares one rate-limit bucket and every abuse report points
at the proxy. Both leave a server that works and is quietly doing the
wrong thing — the worst thing to leave to a README nobody reads.

Three properties, each a way packaging usually goes wrong:

The generated config is NOT a dpkg conffile. It is written from the
debconf answers, so shipping it as one would make dpkg prompt on every
upgrade about changes the package itself had made.

Hand edits survive. postinst rewrites only the keys debconf manages;
comments, ordering and any other setting are left alone.

A blank key on reconfigure keeps the existing one. Otherwise pressing
Enter through a dpkg-reconfigure would unpublish every future upload.

The seeding guard is worth its comment, because the obvious version is
wrong twice over. `config` seeds unanswered questions from the env file
so a reconfigure shows what is actually in force. Seeding
unconditionally overwrites a preseed — debconf-set-selections marks what
it sets as seen — so every unattended install would quietly reconfigure
itself back to whatever was on disk. Guarding on an empty value does not
work either: server-id and bind carry template Defaults, so db_get
returns "localhost" for a question nobody answered. The test is the
`seen` flag, which is the actual question being asked.

Purge keeps the database, knowingly departing from the expectation that
purge removes everything. Manifests are the output of real CV compute on
media the operator may no longer have, and §8 says federation is
explicitly not a backup. Destroying that during an `apt purge` is not a
trade worth making for tidiness; postrm names the path instead.

The nginx example is documentation, not installed configuration. The
proxy usually runs on a different host from the server, so a file
dropped into this machine's nginx would be in the wrong place — and §8
leaves the edge to the operator deliberately.

Verified by running it, not by reading it: a full lifecycle in a
bookworm container — build, preseeded install, mode-600 env file, key
absent from debconf's database afterwards, `systemd-analyze verify` on
the unit, the installed binary answering /health and /ready, reconfigure
preserving both the key and an unmanaged setting, and purge leaving the
database. It failed on the seeding bug above the first time, which is
why that guard exists. CI runs the same checks against every build.

TRACES: DR-015, DR-016 | PR-004

packaging/debian/config

@@ -0,0 +1,66 @@
+#!/bin/sh
+# debconf question script. Runs before unpacking, and again on
+# `dpkg-reconfigure jray-server`.
+#
+# Existing values are read back out of /etc/jray-server/env first, so a
+# reconfigure shows what is actually in force rather than the package defaults.
+# Without this, an operator who edited the env file by hand would be shown stale
+# answers and silently have their edits reverted by postinst.
+set -e
+. /usr/share/debconf/confmodule
+
+ENV_FILE=/etc/jray-server/env
+
+# Read one KEY=value out of the env file, ignoring comments. Values are written
+# unquoted by postinst, so no unquoting is needed.
+env_value() {
+    [ -f "$ENV_FILE" ] || return 0
+    sed -n "s/^$1=//p" "$ENV_FILE" | tail -1
+}
+
+# Seed only a question debconf has never had an answer to.
+#
+# The test is the `seen` flag, not whether the value is empty: two of these
+# templates carry a Default, so db_get returns "localhost" or "127.0.0.1:8080"
+# for a question nobody has answered, and an emptiness check would never seed
+# them. `seen` distinguishes "this is the template default" from "somebody chose
+# this", which is the actual question.
+#
+# And it has to be a guard rather than an unconditional db_set. debconf-set-
+# selections marks what it sets as seen, so an unconditional seed would overwrite
+# a value the operator had just preseeded — every unattended install would
+# quietly reconfigure itself back to whatever was already on disk. Preseeding is
+# the entire point of the unattended path, so debconf wins wherever it has an
+# answer and the env file only fills in what it does not.
+seed() {  # seed <debconf-key> <env-key>
+    db_fget "$1" seen || RET=""
+    [ "$RET" = "true" ] && return 0
+    v=$(env_value "$2")
+    [ -n "$v" ] && db_set "$1" "$v"
+    return 0
+}
+
+seed jray-server/server-id       JRAY_SERVER_ID
+seed jray-server/bind            JRAY_BIND
+seed jray-server/trusted-proxies JRAY_TRUSTED_PROXIES
+seed jray-server/contact         JRAY_CONTACT
+
+# The API key is deliberately NOT seeded back into the prompt: it is a password
+# template, so debconf would render it as a filled-in field the operator cannot
+# read, and accepting it would just rewrite what is already there. Blank means
+# "keep the existing key" and postinst implements exactly that.
+
+db_fget jray-server/publish-peer-directory seen || RET=""
+if [ "$RET" != "true" ] && [ "$(env_value JRAY_PUBLISH_PEER_DIRECTORY)" = "1" ]; then
+    db_set jray-server/publish-peer-directory true
+fi
+
+db_input high   jray-server/server-id            || true
+db_input high   jray-server/bind                 || true
+db_input high   jray-server/trusted-proxies      || true
+db_input high   jray-server/tmdb-api-key         || true
+db_input medium jray-server/contact              || true
+db_input medium jray-server/publish-peer-directory || true
+db_go || true
+
+exit 0