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/app.py

@@ -0,0 +1,67 @@
+from __future__ import annotations
+
+import logging
+from contextlib import asynccontextmanager
+from pathlib import Path
+
+from fastapi import FastAPI
+from fastapi.staticfiles import StaticFiles
+from starlette.types import Scope
+
+import config
+from process_manager import ProcessManager
+from ros_bridge import ROSBridge
+
+logging.basicConfig(level=logging.INFO, format="%(asctime)s %(levelname)s %(name)s: %(message)s")
+logger = logging.getLogger("scan_web.app")
+
+FRONTEND_DIR = Path(__file__).resolve().parent.parent / "frontend"
+
+
+class SessionState:
+    def __init__(self):
+        self.camera_count = config.DEFAULT_CAMERA_COUNT
+        self.recording = False
+        self.recording_path: str | None = None
+        self.recording_started_at: float | None = None
+        self.camera_values: dict = {}  # cam_id -> camera_params.CameraParamValues
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    app.state.pm = ProcessManager()
+    app.state.session = SessionState()
+    app.state.ros = ROSBridge(camera_count=app.state.session.camera_count)
+    app.state.ros.start()
+    logger.info("scan_web backend started (camera_count=%s)", app.state.session.camera_count)
+    try:
+        yield
+    finally:
+        logger.info("shutting down — stopping all managed processes")
+        app.state.pm.stop_all()
+        app.state.ros.stop()
+
+
+app = FastAPI(title="scan_web", lifespan=lifespan)
+
+from routers import system, camera, recording, telemetry  # noqa: E402  (needs app.state types defined above)
+
+app.include_router(system.router, prefix="/api")
+app.include_router(camera.router)
+app.include_router(recording.router, prefix="/api")
+app.include_router(telemetry.router)
+
+class NoCacheStaticFiles(StaticFiles):
+    """Frontend is actively edited during field testing — a browser caching a
+    stale index.html/app.js/style.css (no Cache-Control was set before) led
+    to confusing "it's not applying" reports that were actually just the
+    prior version still running. Static assets here are small and local, so
+    the no-caching cost is negligible."""
+
+    async def get_response(self, path: str, scope: Scope):
+        response = await super().get_response(path, scope)
+        response.headers["Cache-Control"] = "no-store"
+        return response
+
+
+app.mount("/", NoCacheStaticFiles(directory=str(FRONTEND_DIR), html=True), name="frontend")