Add MVP external API with Django Ninja

[Sylthionys]

Summary

Add a small external management API for third-party integrations and fix the Windows uv run dev launcher interpreter selection.

What Changed

• add a disabled-by-default Django Ninja API at /api/v1/
• add Bearer token auth via EXTERNAL_API_ENABLED and EXTERNAL_API_TOKEN
• implement MVP feed endpoints:
  • list
  • detail
  • create
  • patch
  • delete
  • refresh
• implement MVP tag endpoints:
  • list
  • create
  • patch
  • delete
  • replace feed tag set
• keep refresh asynchronous by queueing existing update_multiple_feeds() logic
• hide Ninja docs and OpenAPI routes
• switch the dev launcher to sys.executable so uv run dev uses the project interpreter on Windows

Scope Notes

• feed create/patch only save configuration
• refresh must be called explicitly to queue processing
• no digest API, agent API, filter API, pagination, or frontend/admin redesign

Testing

• added launcher regression test
• added focused API tests for:
  • disabled-by-default behavior
  • auth success/failure
  • feed CRUD
  • duplicate conflict handling
  • tag CRUD
  • tag replacement/clear behavior
  • refresh queueing

Manual Verification

Verified locally:

• POST /api/v1/tags -> 201
• POST /api/v1/feeds -> 201
• POST /api/v1/feeds/{id}/tags -> 200
• POST /api/v1/feeds/{id}/refresh -> 202
• GET /api/v1/feeds/{id} -> 200
• DELETE /api/v1/feeds/{id} -> 204
• DELETE /api/v1/tags/{id} -> 204

Verification

uv run python manage.py test core.tests.test_dev_server core.tests.test_external_api

[Sylthionys]

Additional verification in Docker:

• built a local image from this branch
• verified the container starts successfully
• verified Bearer-authenticated access to /api/v1/feeds
• verified tag creation, feed creation, tag assignment, and refresh via API
• verified refresh completed successfully and updated feed metadata (fetch_status=True, last_fetch populated, subtitle/link fetched)

Note: while testing Docker on Windows, I also found separate Docker build/runtime compatibility issues related to cron installation and CRLF line endings in shell scripts. I did not include those fixes in this PR to keep the scope focused on the external API MVP.

[versun]

@Sylthionys
话说pr前你有使用并测试过吗😂

[Sylthionys]

@Sylthionys 公关前你有使用并测试过吗😂

跑倒是跑过，但确实覆盖得不够完整，感谢补充。🤔

[versun]

@Sylthionys
还有很多边缘条件没有覆盖，我实在没时间补充了，还得麻烦你帮忙多使用测试下，感谢！