docs: mobile & third-party app setup with per-user scoping

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -102,6 +102,35 @@ labels still work; the following sidecar endpoints return an OS error:
 PhotoPrism's `PHOTOPRISM_READONLY` is controlled separately by
 `PP_READONLY` and gates its own backwrite / import paths.
 
+## Mobile & third-party apps (per-user)
+
+PhotoPrism CE does **not** enforce `auth_users.base_path` on API reads —
+any authenticated user can search the whole library. The sidecar
+therefore ships a scoping proxy at `/api/v1/*` (see
+[`sidecar/handlers_ppproxy.go`](sidecar/handlers_ppproxy.go)) and the
+reverse proxy routes the public `/api/v1` there instead of straight to
+PhotoPrism. Result: any PhotoPrism-compatible app pointed at the site
+sees only the logged-in user's photos.
+
+- **Server URL for apps**: the site itself (e.g.
+  `https://photos.hubris.network`). Known-good client:
+  [Gallery for PhotoPrism](https://github.com/Radiokot/photoprism-android-client)
+  (Android/F-Droid).
+- **Login**: the user's normal username/password. For OIDC accounts (no
+  password), mint an app password:
+  `docker exec pp-app photoprism auth add -n "gallery" -s "*" <username>`
+  and use it as the password in the app.
+- **What's scoped**: photo/geo searches, per-photo reads and edits,
+  batch operations, downloads by UID. Hash-addressed media (thumbnails,
+  video streams, file downloads) is token-guarded and passes through.
+- **What's shared** (CE has no per-user variants of these): album
+  *names*, labels, and people — the photos inside them stay scoped.
+  Album zip downloads are generated by PhotoPrism and are not scoped.
+- **Uploads**: the reconciler mirrors `base_path` into `upload_path`,
+  so WebDAV/app uploads land inside the user's own subtree.
+- Sessions with the `admin` role bypass the proxy scoping entirely (the
+  web client's settings/users/index dialogs need the raw API).
+
 ## Dev iteration loop
 
 For fast iteration on the sidecar without rebuilding its image on every