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/filter/FilterPill.tsx

@@ -106,17 +106,26 @@ export function FilterPill({
           <span className="font-mono text-[11px] opacity-90">{value}</span>
         )}
         {isActive && onClear ? (
-          <button
+          <span
+            role="button"
+            tabIndex={0}
             onClick={(e) => {
               e.stopPropagation()
               onClear()
             }}
-            className="ml-0.5 rounded-full p-0.5 hover:bg-primary/30"
+            onKeyDown={(e) => {
+              if (e.key === 'Enter' || e.key === ' ') {
+                e.preventDefault()
+                e.stopPropagation()
+                onClear()
+              }
+            }}
+            className="ml-1 inline-flex h-5 w-5 cursor-pointer items-center justify-center rounded-full hover:bg-primary/30"
             title={`Clear ${label}`}
             aria-label={`Clear ${label}`}
           >
             <X className="h-3 w-3" />
-          </button>
+          </span>
         ) : (
           <ChevronDown className="h-3 w-3 opacity-60" />
         )}

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>

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>