Show fractional star fill for decimal average ratings

[bpepple]

Summary

• Average ratings (e.g. 3.2) previously rendered whole stars only, rounding down and coloring 3 of 5 stars for a 3.2 average with no visual indication of the remaining .2
• Adds a star_fill_percent template filter (comicsdb/templatetags/rating_tags.py) that computes a 0-100% fill for each star position given a decimal average
• Updates partials/rating_summary.html (issue/series detail headers) and the "Average Rating" display in partials/rating_widget.html (issue detail page) to render each star as a grey outline with a gold overlay clipped to that percentage, so a 3.2 average now shows stars 1-3 fully filled, star 4 at ~20%, and star 5 empty
• The interactive "Your Rating" stars in rating_widget.html are unchanged — user ratings are always whole integers (1-5)