ci: check generated schema and default config artifacts (#825)

## ℹ️ Description
- Link to the related issue(s): N/A
- Add a CI guard that fails when generated artifacts are out of sync,
motivated by preventing missing schema updates and keeping generated
reference files current.
- Add a committed `docs/config.default.yaml` as a user-facing default
configuration reference.

## 📋 Changes Summary
- Add `scripts/check_generated_artifacts.py` to regenerate schema
artifacts and compare tracked outputs (`schemas/*.json` and
`docs/config.default.yaml`) against generated content.
- Run the new artifact consistency check in CI via
`.github/workflows/build.yml`.
- Add `pdm run generate-config` and `pdm run generate-artifacts` tasks,
with a cross-platform-safe delete in `generate-config`.
- Add generated `docs/config.default.yaml` and document it in
`docs/CONFIGURATION.md`.
- Update `schemas/config.schema.json` with the
`diagnostics.timing_collection` property generated from the model.

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [ ] 🐞 Bug fix (non-breaking change which fixes an issue)
- [x] ✨ New feature (adds new functionality without breaking existing
usage)
- [ ] 💥 Breaking change (changes that might break existing user setups,
scripts, or configurations)

## ✅ Checklist
Before requesting a review, confirm the following:
- [x] I have reviewed my changes to ensure they meet the project's
standards.
- [x] I have tested my changes and ensured that all tests pass (`pdm run
test`).
- [x] I have formatted the code (`pdm run format`).
- [x] I have verified that linting passes (`pdm run lint`).
- [x] I have updated documentation where necessary.

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Added a reference link to the default configuration snapshot for
easier access to baseline settings.

* **Chores**
* Added a CI build-time check that validates generated schemas and the
default config and alerts when regeneration is needed.
* Added scripts to generate the default config and to sequence artifact
generation.
* Added a utility to produce standardized schema content and compare
generated artifacts.
  * Minor tweak to schema generation success messaging.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

pyproject.toml

@@ -88,6 +88,8 @@ debug = "python -m pdb -m kleinanzeigen_bot"
 
 # build & packaging
 generate-schemas = "python scripts/generate_schemas.py"
+generate-config = { shell = "python -c \"from pathlib import Path; Path('docs/config.default.yaml').unlink(missing_ok=True)\" && python -m kleinanzeigen_bot --config docs/config.default.yaml create-config" }
+generate-artifacts = { composite = ["generate-schemas", "generate-config"] }
 compile.cmd = "python -O -m PyInstaller pyinstaller.spec --clean --workpath .temp"
 compile.env = {PYTHONHASHSEED = "1", SOURCE_DATE_EPOCH = "0"}  # https://pyinstaller.org/en/stable/advanced-topics.html#creating-a-reproducible-build