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>

webapp/services/scenario_scanner.py

@@ -0,0 +1,84 @@
+"""Discover scenario YAML files that can be launched from the console.
+
+Scanning is intentionally tolerant: a malformed scenario file is reported with
+an error string rather than aborting the whole listing, so the UI can show the
+user which files are runnable and which need fixing.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import yaml
+
+from webapp.models import ScenarioInfo
+
+
+def _repo_root() -> Path:
+    """Return the siemens_ragas repository root (parent of the webapp package)."""
+    return Path(__file__).resolve().parents[2]
+
+
+def _scenarios_root() -> Path:
+    """Return the conventional scenarios/ directory inside the repository."""
+    return _repo_root() / "scenarios"
+
+
+def _summarize_scenario(path: Path) -> ScenarioInfo:
+    """Read a scenario file into a compact info object, capturing parse errors."""
+    relative = path.relative_to(_repo_root()).as_posix()
+    try:
+        payload = yaml.safe_load(path.read_text(encoding="utf-8")) or {}
+    except (OSError, yaml.YAMLError) as exc:
+        return ScenarioInfo(path=relative, error=f"无法解析: {exc}")
+
+    if not isinstance(payload, dict):
+        return ScenarioInfo(path=relative, error="场景文件格式不是 YAML 映射。")
+
+    metrics = payload.get("metrics")
+    metric_list = [str(item) for item in metrics] if isinstance(metrics, list) else []
+
+    return ScenarioInfo(
+        path=relative,
+        scenario_name=str(payload.get("scenario_name", "")),
+        mode=str(payload.get("mode", "")),
+        dataset=str(payload.get("dataset", "")),
+        judge_model=str(payload.get("judge_model", "")),
+        metrics=metric_list,
+    )
+
+
+def list_scenarios() -> list[ScenarioInfo]:
+    """Return every scenario YAML under scenarios/, sorted by path."""
+    root = _scenarios_root()
+    if not root.is_dir():
+        return []
+
+    scenarios: list[ScenarioInfo] = []
+    for path in sorted(root.rglob("*.yaml")):
+        scenarios.append(_summarize_scenario(path))
+    for path in sorted(root.rglob("*.yml")):
+        scenarios.append(_summarize_scenario(path))
+    return scenarios
+
+
+def resolve_scenario_path(relative_or_absolute: str) -> Path | None:
+    """Resolve a user-supplied scenario path safely within the repository.
+
+    Only paths that live inside the repository's scenarios/ directory are
+    accepted, which prevents the trigger endpoint from reading arbitrary files.
+    """
+    root = _repo_root()
+    candidate = Path(relative_or_absolute)
+    resolved = candidate if candidate.is_absolute() else (root / candidate)
+    try:
+        resolved = resolved.resolve()
+    except OSError:
+        return None
+
+    scenarios_root = _scenarios_root().resolve()
+    if scenarios_root not in resolved.parents and resolved != scenarios_root:
+        return None
+    if not resolved.is_file():
+        return None
+    return resolved