fix: collision names must survive the sanitiser the content view applies

unique_filename produced 'manual (2).pdf'; HouseplanContentView sanitises the
name in the REQUEST too, turning ' (2)' into '_2_', so the file was written and
then 404'd. The same pattern was already in files/migrate, so a rebind that hit
a name collision has been producing dead links. Both use the shared helper now,
with '-2', which round-trips sanitize_filename — asserted.

tests_backend/test_ha_websocket.py

@@ -690,7 +690,7 @@ async def test_upload_never_overwrites_an_existing_attachment(
     names = sorted(
         n for n in await hass.async_add_executor_job(os.listdir, folder) if n.startswith("manual")
     )
-    assert names == ["manual (2).pdf", "manual.pdf"]
+    assert names == ["manual-2.pdf", "manual.pdf"]
 
     got = await http.get(first.replace(CONTENT_URL, CONTENT_URL))
     assert await got.read() == b"ONE", "the first file is untouched"