Add auto-generated table of contents for posts

[x3ek]

Description

Auto-generate a table of contents (TOC) from post headings and make it available to themes as a template variable.

Split from #9 to keep scope manageable — #9 now covers RSS feed, meta tags, and reading time only.

Requirements

• Parse h2 and h3 headings from rendered markdown to build a nested TOC
• Expose as post.toc (HTML fragment) in Jinja2 templates
• Themes can render it wherever they choose (sidebar, inline, etc.)
• Support frontmatter flag toc: false to disable per-post
• Optionally support a global config toggle in config.yml
• Heading anchors should use slugified heading text (e.g., #my-heading)

Implementation Notes

• Option A: Use python-markdown's toc extension (markdown.extensions.toc) — generates TOC and adds anchor IDs to headings in one pass
• Option B: Post-process rendered HTML with BeautifulSoup/regex to extract headings
• Option A is preferred — it integrates cleanly with the existing markdown pipeline in src/squishmark/services/markdown.py
• Add toc field to the Post model in src/squishmark/models/content.py
• Themes that don't use post.toc are unaffected (variable is simply ignored)