Add search, filter, and sort functionality to film shelves

- Add _SORT_COLUMNS dict to routers/films.py with 8 sort options
- Extend _get_shelf_query to accept q (search) and sort parameters
- Update /films/partial endpoint to accept q/sort query params and pass
  search_active to template to suppress month grouping when searching
- Add filter bar (search input + sort select) to templates/index.html
- Add data-shelf attribute to #film-feed for JS to read current shelf
- Rewrite infinite scroll JS to support debounced search (300ms),
  feed reset on filter/sort change, and pass params on all fetches

Filters text search by title OR director (case-insensitive ilike).
Sort options: date watched (newest/oldest), title (A-Z/Z-A), year,
stars. Month grouping disabled when search is active.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

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