Reels pre-gen: always render the agentic reel, don't adopt on-demand mp4

Past the key-aware dedup, any mp4 already at the cache key was not
pre-generated by us (no matching ledger row) — typically an on-demand
fast-scripted reel sharing the key after the max_segments alignment.
Adopting it recorded a ledger row pointing at the fast reel, silently
defeating agentic pre-gen. Drop the adopt-existing-mp4 shortcut and
always produce_reel (atomic overwrite). Worst case is one redundant
re-render if a prior run crashed between render and ledger write.

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

src/reels/mod.rs

@@ -1121,44 +1121,13 @@ async fn pregen_one(
         return Ok(());
     }
 
-    // Check if MP4 already on disk (from a previous run that crashed after render)
-    let mp4_path = reel_mp4_path(app_state, &key);
-    if mp4_path.exists() {
-        log::info!(
-            "Precomputed reel MP4 already exists for key={}, recording ledger and skipping render",
-            key
-        );
-        // Read title from sidecar if available
-        let sidecar_path = mp4_path.with_extension("json");
-        let title = if sidecar_path.exists() {
-            let sidecar = tokio::fs::read_to_string(&sidecar_path).await.ok();
-            sidecar
-                .and_then(|s| serde_json::from_str::<ReelSidecar>(&s).ok())
-                .map(|s| s.title)
-                .unwrap_or_else(|| format!("{} reel", span))
-        } else {
-            format!("{} reel", span)
-        };
-        let mut reel_dao = app_state.precomputed_reel_dao.lock().expect("lock");
-        reel_dao.record_reel(
-            &ctx,
-            &crate::database::models::InsertablePrecomputedReel {
-                span: span.to_string(),
-                library_key: library.to_string(),
-                cache_key: key.clone(),
-                output_path: mp4_path.to_string_lossy().to_string(),
-                title,
-                media_count,
-                render_version: RENDER_VERSION as i32,
-                tz_offset_minutes: tz,
-                voice: voice.clone(),
-                generated_at: now,
-            },
-        )?;
-        return Ok(());
-    }
-
-    // Generate the reel
+    // Past the key-aware dedup above, any MP4 already at this key was NOT
+    // pre-generated by us (it has no matching ledger row) — most likely an
+    // on-demand fast-scripted reel that happens to share the key. Don't adopt
+    // it: regenerate so the precomputed reel is the agentic one. produce_reel
+    // publishes atomically, overwriting whatever is there. (The narrow
+    // render-succeeded-but-ledger-write-failed crash window just costs one
+    // redundant re-render next run.)
     log::info!("Generating precomputed reel for span={}, key={}", span, key);
     let (title, mp4) = produce_reel(
         app_state,