fix(merge): keep earliest created_at when merging scenes (no false NEW)

The NEW badge keys off scene.created_at. merge_scenes kept the survivor's created_at,
but the dedup caller may pick the freshly re-ingested mirror as keep_id — so deduplicated
old content got a recent created_at and showed up as NEW (report f17799b3). Coalesce to
min(keep, drop) so a merged scene keeps its first-seen date.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/resolve/scene_merge.py

@@ -221,6 +221,12 @@ def _coalesce_canonical_fields(keep: Scene, drop: Scene) -> None:
     if drop.title and len(drop.title) > len(keep.title or ""):
         keep.title = drop.title
         keep.title_normalized = drop.title_normalized
+    # created_at = najwcześniejsze "first seen" obu scen. NEW badge (apka) bazuje na
+    # created_at; gdy dedup wybierze świeży re-ingest/mirror jako `keep`, stara treść
+    # dostawałaby fałszywe NEW po merdżu (report f17799b3). Bierzemy min → zdeduplikowana
+    # scena zachowuje datę pierwszego pojawienia, nie datę re-ingestu.
+    if drop.created_at and keep.created_at and drop.created_at < keep.created_at:
+        keep.created_at = drop.created_at
 
 
 def _close_pending_candidates(