feat(admin): document contactsinteraction app

Signed-off-by: Christoph Wurst <1374172+ChristophWurst@users.noreply.github.com>

admin_manual/groupware/contactsinteraction.rst

@@ -0,0 +1,81 @@
+=====================
+Contacts Interaction
+=====================
+
+The Contacts Interaction app automatically tracks which people a user has recently interacted with
+and provides this data as a read-only CardDAV address book called **Recently contacted**. This
+enables autocomplete suggestions in sharing dialogs, email composition, calendar invitations and
+other places that query the user's address books — even for people who are not saved as explicit
+contacts.
+
+The app is shipped with Nextcloud and enabled by default. It can be disabled.
+
+How interactions are tracked
+----------------------------
+
+The app listens for ``ContactInteractedWithEvent`` events dispatched by other Nextcloud apps. The
+following apps dispatch this event:
+
+* **File sharing**: When a user creates a share with another local user, an email address or a
+  federated remote user.
+* **Calendar**: When a user shares a calendar with another user.
+* **Mail**: When a user sends an email.
+
+Any app can integrate by dispatching a ``ContactInteractedWithEvent`` with at least one identifier:
+a Nextcloud user ID, an email address or a federated cloud ID.
+
+When an interaction is recorded, the app first checks whether the contacted person already exists in
+one of the user's regular address books. If a match is found, no entry is created in the recently
+contacted address book since the person is already a known contact. Self-interactions (where the
+user interacts with themselves) are also ignored.
+
+For new contacts, a minimal vCard is generated containing:
+
+* ``FN`` (display name): Resolved from the Nextcloud user profile if the person is a local user,
+  falling back to the email address or federated cloud ID.
+* ``EMAIL``: Included when an email address is known.
+* ``CLOUD``: Included when a federated cloud ID is known.
+* ``CATEGORIES``: Set to ``Recently contacted``, which allows the Contacts app to identify entries
+  from this address book and offer users the option to copy them to a regular address book.
+
+If the same person is contacted again, the existing entry's timestamp is updated rather than
+creating a duplicate.
+
+The recently contacted address book
+------------------------------------
+
+Each user's recently contacted address book is accessible via CardDAV at::
+
+    /remote.php/dav/addressbooks/users/{userId}/z-app-generated--contactsinteraction--recent/
+
+The ``z-app-generated`` prefix ensures the address book sorts after user-created address books. Users
+cannot create their own address books with this reserved prefix.
+
+The address book is **read-only** and **not shareable**. Users cannot create, modify or delete
+entries. Entries are only removed automatically by the cleanup job or when a user account is deleted.
+
+The address book is visible in:
+
+* **CardDAV clients**: Any client syncing with the user's Nextcloud (e.g., Thunderbird, macOS
+  Contacts, DAVx5) will see the address book.
+* **Nextcloud Contacts**: The address book and its entries appear in the Contacts UI if the
+  Contacts app is enabled. Contacts from this address book can be copied to a regular address book.
+* **Autocomplete**: Entries are available for recipient suggestions in sharing dialogs and other
+  places across the Nextcloud web interface.
+
+Data retention
+--------------
+
+A background job runs every 24 hours and removes entries that have not been updated within the last
+7 days. Both the retention period and the cleanup interval are fixed and cannot be configured.
+
+.. note::
+
+    The cleanup job depends on the Nextcloud background job system. Make sure cron is configured
+    correctly for your instance. See :doc:`../configuration_server/background_jobs_configuration`.
+
+User deletion
+-------------
+
+When a user account is deleted, all of that user's recently contacted entries are automatically
+removed from the database.

admin_manual/groupware/index.rst

@@ -8,6 +8,7 @@ Groupware
 
     calendar
     contacts
+    contactsinteraction
     mail
     out_of_office
     troubleshooting