Add a field for AI policy compliance in the issues/PRs templates (#5803)

* Convert issue templates to Issue Forms with AI policy disclosure

Convert the 3 markdown issue templates (.md) to GitHub Issue Forms (.yml)
to enable interactive form elements, and add an AI disclosure section per
the AI Policy (AI_POLICY.md):

- Required checkbox to acknowledge having read the AI Policy
- Required dropdown for AI usage (none / fully / partially with explanation)
- Optional input for model(s) used (open-weight examples in placeholder)
- Optional input for tool(s) used (FOSS examples in placeholder)
- Optional textarea to explain how AI was used (for the partial option)

The existing template content and structure is preserved, only converted
from markdown to the Issue Forms schema.

* Align pull request template with AI policy disclosure

Update the pull request template to match the AI disclosure standards
introduced in the issue templates:

- Use relative link to AI_POLICY.md instead of absolute GitHub URL
- Update dropdown wording to match issue forms
- Use open-weight models in placeholder examples
- Use FOSS tools in placeholder examples
- Add 'How was AI used?' section for the partial-usage explanation

* Fix assignees field type in issue templates

Use an empty array instead of an empty string for the assignees field,
as required by the GitHub Issue Forms schema.

* Enhance the PR template

* Reword the issue templates to be consistent with the PR template

* Enhance the PR template

* Typo coming from the original template

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Better wording for what comes from the original templates

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Switch to full URLs since relative URL break inconsistently

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+## Checklist
+
+- [] I have read the [AI Policy](https://github.com/iv-org/invidious/blob/master/AI_POLICY.md) and understand the disclosure requirements
+
+
+## AI Disclosure
+
+<!-- Tick exactly one -->
+
+- [] AI was not used to write this pull request
+- [] AI was used to fully write this pull request
+- [] AI was used to partially create this pull request (explain how)
+
+<!-- Leave this whole section blank if you didn't use AI -->
+
+**Model(s) used:**
+<!-- e.g. Llama 3.1, Mistral, Qwen 2.5, DeepSeek-R1 -->
+
+**Tool(s) used:**
+<!-- e.g. Ollama, Open WebUI, Continue -->
+
+**How was AI used?**
+<!-- If you selected "AI was used to partially create this pull request (explain how)" above, explain how AI was used here. Leave empty otherwise. -->
+
+
+## Pull request description