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/static/js/api.js

@@ -0,0 +1,46 @@
+// api.js — 控制台后端 HTTP 接口的轻量封装。
+
+const API = {
+  // 通用 JSON GET，失败时抛出带状态码的错误。
+  async get(path) {
+    const resp = await fetch(path);
+    if (!resp.ok) {
+      const detail = await API._extractError(resp);
+      throw new Error(detail);
+    }
+    return resp.json();
+  },
+
+  // 通用 JSON POST。
+  async post(path, body) {
+    const resp = await fetch(path, {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify(body || {}),
+    });
+    if (!resp.ok) {
+      const detail = await API._extractError(resp);
+      throw new Error(detail);
+    }
+    return resp.json();
+  },
+
+  // 从错误响应中尽量解析出 detail 文本。
+  async _extractError(resp) {
+    try {
+      const data = await resp.json();
+      return data.detail || `请求失败 (${resp.status})`;
+    } catch (_e) {
+      return `请求失败 (${resp.status})`;
+    }
+  },
+
+  health() { return API.get("/api/health"); },
+  runs() { return API.get("/api/runs"); },
+  runDetail(runId) { return API.get(`/api/runs/${encodeURIComponent(runId)}`); },
+  scenarios() { return API.get("/api/scenarios"); },
+  triggerEvaluation(scenarioPath) {
+    return API.post("/api/evaluations", { scenario_path: scenarioPath });
+  },
+  taskStatus(taskId) { return API.get(`/api/evaluations/${encodeURIComponent(taskId)}`); },
+};