Picker: render existing annotation as a clickable mark below the video

When a video that already has a barrier_opening row is loaded, a thin
bar now appears below the player with a red vertical line at the
marked opening_s. A tooltip shows the timestamp, the analyst's
initials, and the bad_rois flag. Clicking the line snaps the player
to the exact opening; clicking elsewhere on the bar seeks
proportionally. Bar is hidden for un-annotated videos.

API change: each queue item now carries an `existing` field with the
opening_s, bad_rois, analyst, etc. from barrier_opening.csv when
available, so the frontend can paint the mark without extra requests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/barrier_picker_app/app.py

@@ -73,6 +73,7 @@ class QueueItem:
     duration_s: float | None
     done: bool
     metadata: dict   # experimental fields aggregated from the merged TSV
+    existing: dict | None   # current barrier_opening.csv row if already picked
 
 
 # ─── Queue building ─────────────────────────────────────────────────────
@@ -142,13 +143,19 @@ def _build_queue() -> list[QueueItem]:
             "duration_s": float(r.duration_s) if pd.notna(r.duration_s) else None,
         }
 
+    existing_by_key: dict[tuple[str, str, str], dict] = {}
     if OUTPUT_CSV.exists():
         out = pd.read_csv(OUTPUT_CSV)
-        done_keys = set(zip(out["machine_name"],
-                            out["session_date"],
-                            out["session_time"]))
-    else:
-        done_keys = set()
+        for _, r in out.iterrows():
+            key = (r["machine_name"], r["session_date"], r["session_time"])
+            existing_by_key[key] = {
+                "opening_s": float(r["opening_s"]) if pd.notna(r["opening_s"]) else None,
+                "trim_first_s": int(r["trim_first_s"]) if pd.notna(r.get("trim_first_s")) else 0,
+                "bad_rois": str(r.get("bad_rois", "") or "") if pd.notna(r.get("bad_rois")) else "",
+                "analyst": str(r.get("analyst", "") or "") if pd.notna(r.get("analyst")) else "",
+                "notes": str(r.get("notes", "") or "") if pd.notna(r.get("notes")) else "",
+            }
+    done_keys = set(existing_by_key)
 
     seen: set[tuple[str, str, str]] = set()
     items: list[QueueItem] = []
@@ -189,6 +196,7 @@ def _build_queue() -> list[QueueItem]:
                 duration_s=inv_row["duration_s"],
                 done=key in done_keys,
                 metadata=metadata,
+                existing=existing_by_key.get(key),
             ))
     return items
 
@@ -217,6 +225,7 @@ async def get_queue() -> JSONResponse:
             "duration_s": q.duration_s,
             "done": q.done,
             "metadata": q.metadata,
+            "existing": q.existing,
         }
         for q in queue
     ])