Rename iPodderX to iPX (#56)

The app, the repository (rays/ipx), the image, the compose service and
container, and the data folders on Tower take the new name. What stays:
the 2004 iPodderX and ipodderx-core, which are history; the Postgres
database and login, and ipodderx.sdf1.net with its Access and Authentik
apps, which would each need moving outside this repo; and the first-start
password, as ipx is shorter than the eight characters a password needs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -1,4 +1,4 @@
-# ipodderx-rs
+# ipx
 
 A self-hosted podcatcher for a household. It checks your feeds, downloads the episodes, and serves
 a web UI modelled on the 2004 Mac app **iPodderX**, for any number of people sharing one copy of
@@ -30,7 +30,7 @@ behind iPodderX (2004-2008, Ray Slakinski & August Trometer).
 With Docker:
 
 ```sh
-docker build -t ipodderx .
+docker build -t ipx .
 docker compose up -d
 ```