Implement songs pipeline v1

- render.py: frontmatter validation, h1 strip, pandoc conversion,
  banner/category injection, idempotent MediaWiki writes via mwclient
- requirements.txt: python-frontmatter, mwclient, PyYAML
- template_auto_generated.wikitext: paste into wiki as Template:Auto-generated
- .gitea/workflows/publish-songs.yml: reusable workflow; job container
  python:3.12-slim installs pandoc via apt (no host-level pandoc needed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pipelines/songs/template_auto_generated.wikitext

@@ -0,0 +1,9 @@
+{{#if:{{{source|}}}|<div style="background:#f8f9fa;border:1px solid #a2a9b1;padding:0.4em 0.8em;margin-bottom:1em;font-size:0.85em;color:#54595d;">&#x26A0;&#xFE0F; This page is automatically published from source. Manual edits will be overwritten on the next pipeline run. &nbsp;&bull;&nbsp; Source: [{{{source}}} view on Gitea] &nbsp;&bull;&nbsp; Generated: {{{generated_at}}}</div>}}
+<noinclude>
+== Usage ==
+Applied automatically by the songs pipeline. Parameters:
+* <code>source</code> — URL to the source file on Gitea
+* <code>generated_at</code> — ISO 8601 timestamp of the publishing run
+
+[[Category:Auto-generated templates]]
+</noinclude>