Rename the import package app to fluksio

A wheel whose top-level module is `app` collides with anything else in a
user's venv, so the package that is about to be published takes the name
it is published under. Only the Python package moves; the repo, the
Docker WORKDIR and the compose project keep theirs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.pre-commit-config.yaml

@@ -13,7 +13,7 @@ repos:
         exclude: |
             (?x)^(
                 frontend/src/client/.*|
-                backend/app/email-templates/build/.*
+                backend/fluksio/email-templates/build/.*
             )$
       - id: trailing-whitespace
         exclude: ^frontend/src/client/.*