The first admin gets a random password, not admin / ipodderx (#321)

A database with no accounts created admin with the hard-coded password
ipodderx, published in the README, the docs and the sample compose
file, and the log line announcing it printed the password. /api/login
is open to anyone who reaches the port, and nothing made anyone change
it, so a fresh install was an admin account anyone could sign into.

Now the password is IPX_ADMIN_PASSWORD when set, for scripted installs,
or 24 random hex characters written to admin-password beside
config.toml, created 0600 so it is never readable by others even for a
moment. The log says where it is, never what it is, as it does for the
web token (#40). Existing databases are untouched: seeding runs only
with no accounts at all.

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

docs/configuration.md

@@ -183,6 +183,7 @@ had it only through the OPML loses it as before. Leaving the OPML yourself keeps
 | Variable | Effect |
 |---|---|
 | `IPX_CONFIG` | Config file path |
+| `IPX_ADMIN_PASSWORD` | The first admin's password, for a database with no accounts; random, in `admin-password` beside config.toml, without it |
 | `IPX_DATA_DIR` | Directory holding `state.db` |
 | `IPX_DATABASE_URL` | A `postgres://user:password@host:port/database` URL: use that database instead of `state.db` |
 | `IPX_TEST_DATABASE_URL` | For `cargo test`: run the database tests on this Postgres database too, each in a schema of its own |