fix: respect provider for image analysis

[Endorpheen]

Summary

• route /image/analyze through the active provider (OpenRouter vs OpenAI Compatible) on both frontend and backend
• adjust UI validations so image hints depend on the selected provider only
• add regression tests for image analysis provider routing and payloads

Bug Reproduction

1. Select "OpenAI Compatible" in settings.
2. Upload an image and ask "что на изображении?".
3. UI requires OpenRouter API key and backend always calls OpenRouter.

Fix Verification

• OpenAI Compatible image analysis now uses its base URL/API key, without OpenRouter prompts.
• OpenRouter behaviour unchanged; provider switches do not reset the form.
• Tests: PYTHONPATH=. uv run pytest, npm run test:backend, npm --prefix web-ui run lint, npm run test:frontend.

Screenshots

• TODO (CLI environment; attach after UI verification)