fix: more audit findings — perf, types, and a11y polish

- backend/photos: collapse the per-tag subquery loop in the tag filter
  into a single GROUP BY ... HAVING COUNT(DISTINCT) = N subquery so the
  cost is independent of how many tags the user is filtering on.
- useFilterUrlSync: type the parseUrl return value as
  Partial<FilterState> & { currentSection?: string } so the section field
  doesn't need an (out as any) cast.
- Timeline sticky header: bump opacity, padding, and border so it reads
  more clearly against the underlying grid.
- FilterPill clear: convert the nested <button> (invalid HTML — buttons
  cannot nest) to a span with role=button + keyboard handler, with a
  larger hit area.
- RightSidebar: add aria-label to the close-X buttons so screen readers
  announce them.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/timeline/Timeline.tsx

@@ -422,7 +422,7 @@ export function Timeline() {
        *  positioned children so it isn't affected by translateY transforms.
        *  Updates as the user scrolls past month boundaries. */}
       {stickyLabel && (
-        <div className="pointer-events-none absolute left-0 right-0 top-0 z-20 border-b border-border bg-bg/90 px-4 py-1 backdrop-blur-sm">
+        <div className="pointer-events-none absolute left-0 right-0 top-0 z-20 border-b-2 border-border bg-bg/95 px-4 py-1.5 shadow-sm backdrop-blur">
           <h3 className="text-sm font-semibold uppercase tracking-wide text-text">
             {stickyLabel}
           </h3>