refactor: Migrate to sphinxnotes-render

[SilverRainZ]

• Replace custom directive and Jinja2 Environment with render's extra context
• Register @extra_context('recentupdates') accepting *args, **kwargs for count
• Register @filter('strftime') via render's filter system
• roles filter now provided by render, removing duplicate
• Drop recentupdate_count and recentupdate_template config values
• Remove RecentUpdateDirective and custom Environment class
• Update docs to show new load_extra('recentupdates', 10) usage