Refactor internal addon references to app/apps

Rename all internal code references from addon/addons to app/apps
to align with the rebranding initiative. This covers:
- Variable and argument names
- Function and method names
- Class names and attributes
- Docstrings and comments

External API contracts (JSON keys, error codes, discovery protocol
fields) are preserved unchanged. File and directory names under
supervisor/addons/ are also unchanged as those are filesystem paths.

Additionally fixes the backup/restore partial API endpoints which
use **body to forward request data to do_backup_partial and
do_restore_partial. Since the external API key remains addons
(ATTR_APPS = "addons") but the internal parameter was renamed to
apps, a remapping step is added before the **body expansion.
A test for partial restore with the addons request key is also
added to cover this path.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mdegat01

supervisor/addons/__init__.py

@@ -1 +1 @@
-"""Init file for Supervisor add-ons."""
+"""Init file for Supervisor apps."""