Initial commit: scan_web integrated recording UI

FastAPI backend + vanilla JS frontend for LiDAR/camera startup, camera
settings, and rosbag recording, unifying the previous scan_gui/scan_gui_dual/
scan_gui_triple desktop tools into one web app.

backend/bag_metadata.py

@@ -0,0 +1,55 @@
+"""Read back what ros2 bag already knows about a recording — duration,
+message counts, per-topic counts — instead of just listing directory names.
+Also a plain on-disk size sum (metadata.yaml doesn't carry a byte size in
+the ros2 bag version installed here)."""
+from __future__ import annotations
+
+import os
+from pathlib import Path
+from typing import Optional
+
+import yaml
+
+
+def dir_size_bytes(path: Path) -> int:
+    total = 0
+    try:
+        for entry in path.iterdir():
+            if entry.is_file():
+                total += entry.stat().st_size
+    except OSError:
+        pass
+    return total
+
+
+def read_bag_metadata(bag_dir: Path) -> Optional[dict]:
+    """Returns None if metadata.yaml is missing (e.g. bag was killed mid-write
+    before rosbag2 flushed it) or unparseable — a real, if incomplete, state
+    the operator should be able to see rather than a crash."""
+    meta_path = bag_dir / "metadata.yaml"
+    if not meta_path.exists():
+        return None
+    try:
+        with open(meta_path, encoding="utf-8") as f:
+            raw = yaml.safe_load(f)
+        info = raw["rosbag2_bagfile_information"]
+    except Exception:
+        return None
+
+    duration_s = info.get("duration", {}).get("nanoseconds", 0) / 1e9
+    message_count = info.get("message_count", 0)
+    topics = []
+    for entry in info.get("topics_with_message_count", []):
+        tm = entry.get("topic_metadata", {})
+        topics.append({
+            "name": tm.get("name", "?"),
+            "type": tm.get("type", "?"),
+            "message_count": entry.get("message_count", 0),
+        })
+
+    return {
+        "duration_s": round(duration_s, 1),
+        "message_count": message_count,
+        "topics": topics,
+        "size_bytes": dir_size_bytes(bag_dir),
+    }