refactor: move package source to src/ layout

Moves sample_plugin/ into src/ to adopt the standard src-layout, which
prevents accidental imports from the source tree during tests and fixes
editable-install compatibility with mypy and other tools that rely on
proper package discovery.

- backend/sample_plugin/ moved to backend/src/sample_plugin/
- backend/pyproject.toml: add `where = ["src"]` to [tool.setuptools.packages.find]
- backend/MANIFEST.in: update recursive-include path, remove stale requirements/ lines
- backend/tox.ini: update quality tool invocations to use src/sample_plugin path

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: feanil

backend/MANIFEST.in

@@ -1,5 +1,3 @@
 include LICENSE.txt
 include README.md
-include requirements/base.in
-include requirements/constraints.txt
-recursive-include sample_plugin *.html *.png *.gif *.js *.css *.jpg *.jpeg *.svg
+recursive-include src/sample_plugin *.html *.png *.gif *.js *.css *.jpg *.jpeg *.svg

backend/pyproject.toml

@@ -95,6 +95,7 @@ dev = [
 readme = {file = ["README.md"], content-type = "text/markdown" }
 
 [tool.setuptools.packages.find]
+where = ["src"]
 include = ["sample_plugin*"]
 exclude = ["sample_plugin.tests*"]