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/log_tail.py

@@ -0,0 +1,54 @@
+"""Tails the lidar/camera/recording log files for the browser log panel.
+
+The original PyQt5 scan GUIs had a single unified QPlainTextEdit showing all
+subprocess stdout with a [tag] prefix — this is the web equivalent. Each
+ManagedProcess re-opens its log file in "w" mode on every start (see
+process_manager.py), so a tailer must detect truncation (file shrank since
+last read) and reset to the top rather than seeking past EOF forever.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Iterable
+
+
+class LogTailer:
+    def __init__(self, sources: dict[str, Path]):
+        self._sources = sources
+        self._offsets: dict[str, int] = {tag: 0 for tag in sources}
+
+    def read_tail(self, tag: str, max_lines: int = 200) -> list[str]:
+        """Backlog for a fresh websocket connection — last N lines, and
+        advances this tag's offset to end-of-file so read_new() only
+        reports genuinely new lines afterwards."""
+        path = self._sources.get(tag)
+        if path is None or not path.exists():
+            return []
+        try:
+            with open(path, "r", errors="replace") as f:
+                lines = f.readlines()
+                self._offsets[tag] = f.tell()
+        except OSError:
+            return []
+        return [ln.rstrip("\n") for ln in lines[-max_lines:]]
+
+    def read_new(self) -> Iterable[tuple[str, str]]:
+        """Yields (tag, line) for every line appended since the last call."""
+        for tag, path in self._sources.items():
+            if not path.exists():
+                continue
+            try:
+                size = path.stat().st_size
+                if size < self._offsets.get(tag, 0):
+                    self._offsets[tag] = 0  # file was truncated (process restarted)
+                with open(path, "r", errors="replace") as f:
+                    f.seek(self._offsets[tag])
+                    new_text = f.read()
+                    self._offsets[tag] = f.tell()
+            except OSError:
+                continue
+            if not new_text:
+                continue
+            for line in new_text.splitlines():
+                if line:
+                    yield tag, line