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>

rag_eval/datasets/loader.py

@@ -0,0 +1,56 @@
+"""Load raw evaluation dataset records from disk.
+
+Supports CSV and JSONL formats. Returns a list of plain dicts — normalization
+into NormalizedSample is handled by normalizers.py.
+"""
+
+from __future__ import annotations
+
+import csv
+import json
+from pathlib import Path
+from typing import Any
+
+
+def load_dataset_records(path: Path | str) -> list[dict[str, Any]]:
+    """Load raw records from a CSV or JSONL file.
+
+    Each row becomes a plain dict. Lists stored as JSON strings in CSV columns
+    are left as-is; normalizers handle parsing.
+    """
+    file_path = Path(path)
+    if not file_path.is_file():
+        raise FileNotFoundError(f"Dataset file not found: {file_path}")
+
+    suffix = file_path.suffix.lower()
+    if suffix in (".jsonl", ".ndjson"):
+        return _load_jsonl(file_path)
+    if suffix in (".csv",):
+        return _load_csv(file_path)
+    # Fall back to CSV for unknown extensions.
+    return _load_csv(file_path)
+
+
+def _load_csv(path: Path) -> list[dict[str, Any]]:
+    """Read a CSV file into a list of row dicts."""
+    with path.open(encoding="utf-8", newline="") as fh:
+        reader = csv.DictReader(fh)
+        return [dict(row) for row in reader]
+
+
+def _load_jsonl(path: Path) -> list[dict[str, Any]]:
+    """Read a JSONL file into a list of record dicts."""
+    records: list[dict[str, Any]] = []
+    with path.open(encoding="utf-8") as fh:
+        for lineno, line in enumerate(fh, 1):
+            line = line.strip()
+            if not line:
+                continue
+            try:
+                obj = json.loads(line)
+            except json.JSONDecodeError as exc:
+                raise ValueError(f"Invalid JSON on line {lineno} of {path}: {exc}") from exc
+            if not isinstance(obj, dict):
+                raise ValueError(f"Expected JSON object on line {lineno} of {path}, got {type(obj).__name__}")
+            records.append(obj)
+    return records