feat(memories): optional reference_date to browse relative to another day

GET /memories accepts reference_date=YYYY-MM-DD (a day in the owner's
display zone). It re-anchors both the span match (day_in_span's "today")
and the pre-filter window, which ends at the end of that day, so the
20-year lookback is measured from it and a future date surfaces
everything taken so far. Omitted → today, unchanged. Malformed → 400.

Reels stay anchored on now (MVP): the reel selector passes None.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/reels/selector.rs

@@ -296,6 +296,8 @@ pub fn resolve(
                 *span,
                 zone,
                 library.as_deref(),
+                // Reels are always "now" (MVP): no reference date.
+                None,
             )?;
 
             // Split into photos and video clips; anything that's neither is