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/layout/RightSidebar.tsx

@@ -97,6 +97,7 @@ export function RightSidebar() {
             onClick={clearSelection}
             className="rounded p-1 text-text-muted hover:bg-surface-2 hover:text-text"
             title="Clear selection"
+            aria-label="Clear selection"
           >
             <X className="h-4 w-4" />
           </button>