fix: serialize downloaded ad timestamps as schema-compliant strings (#863)

## ℹ️ Description
- Link to the related issue(s): Issue #
- Fixes drift where `pdm run app download` wrote timestamp values in
YAML-native datetime form that could violate `schemas/ad.schema.json`
string expectations.
- Ensures downloaded ads persist `created_on`/`updated_on` as
JSON-serialized ISO-8601 strings and adds a regression test validating
written YAML against the schema.

## 📋 Changes Summary
- Updated downloader save path to use `ad_cfg.model_dump(mode =
\"json\")` before writing YAML in `src/kleinanzeigen_bot/extract.py`.
- Updated existing `download_ad` unit assertion to match JSON-mode
serialization.
- Added `test_download_ad_writes_schema_compliant_yaml` in
`tests/unit/test_extract.py` that writes a real tmp YAML file and
validates it against `schemas/ad.schema.json` with `jsonschema`.
- Added dev dependency `jsonschema>=4.26.0` (and lockfile updates).
- Dependencies/config updates introduced: new dev dependency
(`jsonschema`) for full schema validation in tests.

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


## ✅ Checklist
- [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

# Release Notes

* **Bug Fixes**
* Improved ad data serialization to ensure consistent JSON format when
saving ad configurations.

* **Tests**
  * Added schema validation tests to verify ad YAML output compliance.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/kleinanzeigen_bot/extract.py

@@ -65,7 +65,7 @@ class AdExtractor(WebScrapingMixin):
         header_string = (
             "# yaml-language-server: $schema=https://raw.githubusercontent.com/Second-Hand-Friends/kleinanzeigen-bot/refs/heads/main/schemas/ad.schema.json"
         )
-        await asyncio.get_running_loop().run_in_executor(None, lambda: dicts.save_dict(ad_file_path, ad_cfg.model_dump(), header = header_string))
+        await asyncio.get_running_loop().run_in_executor(None, lambda: dicts.save_dict(ad_file_path, ad_cfg.model_dump(mode = "json"), header = header_string))
 
     @staticmethod
     def _download_and_save_image_sync(url:str, directory:str, filename_prefix:str, img_nr:int) -> str | None: