Documentation: docs/, a changelog, and CLAUDE.md

PROGRESS.md becomes CHANGELOG.md with the finished step lists moved to an
appendix. The README is an overview pointing at docs/: configuration,
cli, users, sso (refreshed for accounts and admin-only settings), and
architecture. CLAUDE.md collects what working on this code actually
requires -- pkill -x not -f, the page being compiled in, the dead columns
on entries, the Playwright worker that deleted its own database.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AdXho5tTkjFLeUXKbEjKBh

docs/users.md

@@ -0,0 +1,83 @@
+# Accounts, and what several people share
+
+ipx serves any number of people from one copy of the data. The rule that decides everything else:
+**there is one file on disk per enclosure URL.** Two people subscribed to the same show cost one
+fetch, one parse and one file.
+
+## What is yours, what is everyone's
+
+| Yours alone | The same for everyone |
+|---|---|
+| Read, starred, playback position | The feed's URL |
+| Which feeds you see at all | Its download folder |
+| Keywords, auto-download, explicit, per-scan cap | When it is scanned |
+| | The file on disk |
+
+The right-hand column describes the feed and the file rather than a preference — two people wanting
+different folders would mean two copies. Those three are **admin-only**, and the API returns `403`
+for anyone else rather than merely hiding the controls.
+
+## How the scanner merges everyone's wants
+
+One fetch serves every subscriber, so the policy is a union:
+
+* an item is downloaded if **anyone** wants it — one person's keyword set matching is enough, and
+  one person with no keywords removes the filter for that feed entirely
+* auto-download is on if **anyone** has it on
+* the per-scan cap is the **largest** anyone asked for
+
+So "auto-download off" means *I don't cause downloads*, not *I never see them*. If someone else's
+subscription pulls an item, you see it listed as downloaded and can play it, because the enclosure
+is shared.
+
+## Deleting
+
+Deleting a file deletes everyone's copy. A feed with other subscribers labels the button **Delete
+for everyone** and names them in the confirmation, and the server has the last word: if anyone else
+has starred the item or not played it yet, `DELETE /api/enclosures/{id}` answers `409` with the
+reason, and only `?force=true` goes through.
+
+Retention follows the same rule: starred by anyone keeps a file, and it counts as read only once
+every subscriber has read it.
+
+## Signing in
+
+Three ways, tried in order of how specific the claim is:
+
+1. **A proxy header** naming the user — Cloudflare Zero Trust or Authentik. Honoured only from an
+   address in `trusted_proxies`. See [sso.md](sso.md).
+2. **A session cookie** from signing in at `/login`. Argon2id hashes, sessions in the database,
+   idle timeout `session_days`.
+3. **The shared `[web] token`**, which signs in as the admin — this is what the Docker healthcheck
+   and any scripts use.
+
+A database with no accounts creates **admin / ipodderx** on the next daemon start and says so in
+the log. Change it:
+
+```sh
+echo -n 'a good password' | ipx user passwd admin
+```
+
+## Adding someone
+
+```sh
+echo -n 'their password' | ipx user add sam
+```
+
+They sign in at `/login` and start with **no feeds**: subscriptions are per person. Adding a feed
+someone else already has costs nothing — no second fetch, no second copy — it just appears on their
+list with their own read state. Unsubscribing removes it from their list alone; only when the last
+subscriber leaves does the feed stop being scanned, and even then its files and history stay, so
+re-subscribing does not pull the back catalogue again.
+
+Accounts are managed from the command line only; there is no user administration in the web UI yet.
+
+## Admin
+
+The first account is an admin. An admin can change global settings (scanning interval, quota,
+retention, media types, download folder) and a feed's URL, folder and schedule. Everyone else gets
+the Settings button hidden and a `403` if they ask anyway.
+
+```sh
+ipx user list      # the admin column says who
+```