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.

custom_components/houseplan/plans.py

@@ -35,7 +35,11 @@ def unique_filename(directory: Path, name: str) -> str:
         stem, suffix = safe, ""
     i = 2
     while True:
-        candidate = f"{stem} ({i}){'.' + suffix if suffix else ''}"
+        # "-2", not " (2)": the content view sanitizes the name in the REQUEST
+        # too, and a space or a bracket there turns into "_", so a file called
+        # "manual (2).pdf" was written and then never served. Only characters
+        # that survive sanitize_filename may be used to build a name.
+        candidate = f"{stem}-{i}{'.' + suffix if suffix else ''}"
         if not (directory / candidate).exists():
             return candidate
         i += 1