fix(template): scope babel.cfg python extraction to src/

[davidpoblador]

Summary

vibetuner-template/babel.cfg previously read:

[python: **.py]
input_paths = src

input_paths = src is not a real pybabel directive inside an extractor section — it's silently ignored as an unknown method keyword. The effective pattern **.py matches every Python file under the walk root passed to pybabel extract, so just extract-translations (which runs pybabel extract -F babel.cfg -o locales/messages.pot .) was pulling translatable strings from tests/, scripts/, and anything else outside src/.

Fix

Scope the python pattern to src/**.py and drop the bogus line:

[python: src/**.py]

[jinja2: templates/frontend/**.html.jinja]
input_encoding = utf-8

Verification

Ran pybabel extract against the template directory before and after:

• Before: walks src/{{ project_slug | replace('-', '_') }}/__init__.py and tests/__init__.py.
• After: walks only src/{{ project_slug | replace('-', '_') }}/__init__.py.

Note for existing scaffolded projects

This file lives in vibetuner-template/, so existing user projects won't auto-update. If you've scaffolded a project from a prior version of the template, manually update your babel.cfg to match (or run a Copier update / just deps-scaffolding if you have that wired up).

Closes #1717

Test plan

• pybabel extract -F vibetuner-template/babel.cfg -o /tmp/test.pot vibetuner-template/ walks only src/-scoped files
• Pre-commit hooks pass