Reels pre-gen: use DEFAULT_MAX_SEGMENTS so cache keys match on-demand

pregen_one hardcoded max_segments: 24 while create_reel_handler defaults
to DEFAULT_MAX_SEGMENTS (40). Since the cache key encodes the raw
max_segments, the pre-generated reel's key never matched the client's
on-demand request, so POST /reels cache-hit an older max=40 reel and the
agentic pre-gen file was left orphaned. Align to DEFAULT_MAX_SEGMENTS (as
the plan specified) so the on-demand cache-hit path serves the pre-gen
reel. Content is unchanged — the actual beat count is duration-budgeted
either way; only the key descriptor differed.

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

src/reels/mod.rs

@@ -1053,7 +1053,11 @@ async fn pregen_one(
         } else {
             Some(library.to_string())
         },
-        max_segments: 24,
+        // Must match the on-demand default (create_reel_handler) so the cache
+        // key — which encodes the raw max_segments — lines up and the on-demand
+        // cache-hit path serves this pre-generated reel. The client sends no
+        // max_segments, so it defaults to DEFAULT_MAX_SEGMENTS there too.
+        max_segments: selector::DEFAULT_MAX_SEGMENTS,
     };
 
     let exif_dao = app_state.insight_generator.exif_dao();