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/docs/conf.py

@@ -522,8 +522,8 @@ def on_init(app):  # pylint: disable=unused-argument
         # If we are, assemble the path manually
         bin_path = os.path.abspath(os.path.join(sys.prefix, 'bin'))
         apidoc_path = os.path.join(bin_path, apidoc_path)
-    check_call([apidoc_path, '-o', docs_path, os.path.join(root_path, 'sample_plugin'),
-                os.path.join(root_path, 'sample_plugin/migrations')])
+    check_call([apidoc_path, '-o', docs_path, os.path.join(root_path, 'src/sample_plugin'),
+                os.path.join(root_path, 'src/sample_plugin/migrations')])
 
 
 def setup(app):

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*"]