Add RAGAS evaluation web console (FastAPI + vanilla JS)

- webapp/: FastAPI backend with runs/scenarios/evaluations API routers;
  services for run_reader, report_builder, scenario_scanner, task_manager
  (lazy ragas import — server boots even without ragas); Pydantic models
- webapp/static/: single-page console (layout A: left-nav + main area);
  report detail with metric cards, Chart.js distribution histogram,
  grouping table, lowest-score sample review; trigger evaluation + log polling
- webmain.py: uvicorn entry point (alongside existing main.py CLI)
- start.bat: Windows one-click launcher with env checks and auto-browser open
- rag_eval/datasets/: implement missing loader + normalizer modules
  (load_dataset_records, normalize_records) required by evaluator
- scripts/seed_sample_run.py: generate realistic demo run artifacts
- .gitignore: exclude datasets/ data files but keep rag_eval/datasets/ source

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

webmain.py

@@ -0,0 +1,42 @@
+"""CLI entry point that launches the evaluation console web server.
+
+Run alongside the existing main.py CLI; both share the same rag_eval library
+and the same runs/ artifacts. Example:
+
+    python webmain.py
+    python webmain.py --host 0.0.0.0 --port 8800
+"""
+
+from __future__ import annotations
+
+import argparse
+
+import uvicorn
+
+
+def parse_args() -> argparse.Namespace:
+    """Parse host/port/reload options for the console server."""
+    parser = argparse.ArgumentParser(description="Launch the RAGAS evaluation console.")
+    parser.add_argument("--host", default="127.0.0.1", help="Bind host (default 127.0.0.1).")
+    parser.add_argument("--port", type=int, default=8800, help="Bind port (default 8800).")
+    parser.add_argument(
+        "--reload",
+        action="store_true",
+        help="Enable auto-reload for local development.",
+    )
+    return parser.parse_args()
+
+
+def main() -> None:
+    """Start uvicorn with the configured application."""
+    args = parse_args()
+    uvicorn.run(
+        "webapp.server:app",
+        host=args.host,
+        port=args.port,
+        reload=args.reload,
+    )
+
+
+if __name__ == "__main__":
+    main()