Stop shipping "hackme" as the Icecast password

The source password had a `hackme` fallback in five places -- entrypoint.sh,
docker-compose.yml, setup.sh's prompt default, .env.example, edge-node's
config.py -- plus op25-container's os.getenv default and two README rows. Any
node whose operator pressed Enter through setup.sh is running a credential
that is written down in this repo.

That matters more than the usual default-password case because Icecast binds
all interfaces and the SOURCE password is write access: it does not just let a
LAN neighbour listen, it lets them PUSH audio into the stream the frontend and
mobile clients play as live radio. Injecting fake traffic into a public-safety
feed is the failure worth preventing.

Approach: remove every fallback rather than change them to a better default.
- icecast/entrypoint.sh refuses to start if either password is empty, and says
  how to generate one. This is the single hard gate; everything else is
  defence in depth behind it.
- docker-compose.yml uses ${VAR:?message} so a missing value stops the stack
  at compose time with a readable error instead of becoming an empty string.
- setup.sh GENERATES a random password when the operator presses Enter, via
  openssl rand -base64 24 with a /dev/urandom fallback. Pressing Enter now
  gives you a random password rather than a known one, which is the actual
  behaviour change -- a prompt default nobody types over is not a default, it
  is the value.
- .env.example ships the keys empty with the generation command in a comment,
  and README.md now marks both as required with no default.

Client suite: 185 passed.

Note this does NOT rotate anything already deployed. node-002's .env still has
whatever it was set up with; that is an operational step, tracked in the issue.

Closes logan/node-26#3

icecast/entrypoint.sh

@@ -1,8 +1,19 @@
 #!/bin/sh
 set -e
 
-ICECAST_SOURCE_PASSWORD="${ICECAST_SOURCE_PASSWORD:-hackme}"
-ICECAST_ADMIN_PASSWORD="${ICECAST_ADMIN_PASSWORD:-admin}"
+# No defaults here on purpose. This container binds all interfaces, so a
+# fallback password is a published credential on every node that ever accepted
+# it -- and the source password is what lets a caller PUSH audio into the
+# stream the frontend plays as live radio. Refuse to start instead.
+for var in ICECAST_SOURCE_PASSWORD ICECAST_ADMIN_PASSWORD; do
+  eval "value=\${$var}"
+  if [ -z "$value" ]; then
+    echo "icecast: $var is not set." >&2
+    echo "icecast: set it in the node's .env -- 'bash setup.sh' generates a random one," >&2
+    echo "icecast: or run: openssl rand -base64 24" >&2
+    exit 1
+  fi
+done
 
 export ICECAST_SOURCE_PASSWORD ICECAST_ADMIN_PASSWORD