feat: add detailed request logging to /api/score and global 422 handler

- Log incoming request (client, content-type, metrics, has_gt) on each /api/score call
- Log scoring result (latency, skipped metrics, scores) on success
- Register global RequestValidationError handler: logs url/content-type/errors
  so 422 causes are visible in server log without checking HTTP response body
- Fix jsonable_encoder for exc.errors() to handle non-serializable ctx objects

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

webapp/server.py

@@ -7,15 +7,19 @@ the server starts even when the evaluation dependencies are not yet installed.
 
 from __future__ import annotations
 
+import logging
 from pathlib import Path
 
-from fastapi import FastAPI
-from fastapi.responses import FileResponse
+from fastapi import FastAPI, Request
+from fastapi.encoders import jsonable_encoder
+from fastapi.exceptions import RequestValidationError
+from fastapi.responses import FileResponse, JSONResponse
 from fastapi.staticfiles import StaticFiles
 
 from webapp.api import evaluations, llm_profiles, pipeline, runs, scenarios, score
 
 STATIC_DIR = Path(__file__).resolve().parent / "static"
+logger = logging.getLogger("webapp.server")
 
 # OpenAPI tag metadata — controls the grouping and descriptions in /docs.
 OPENAPI_TAGS = [
@@ -103,6 +107,21 @@ def create_app() -> FastAPI:
     app.include_router(pipeline.router)
     app.include_router(score.router)
 
+    @app.exception_handler(RequestValidationError)
+    async def validation_exception_handler(request: Request, exc: RequestValidationError) -> JSONResponse:
+        """Log full validation error detail to help diagnose 422 responses."""
+        errors = jsonable_encoder(exc.errors())
+        logger.warning(
+            "[422] validation error  url=%s  content_type=%s  errors=%s",
+            request.url.path,
+            request.headers.get("content-type", ""),
+            errors,
+        )
+        return JSONResponse(
+            status_code=422,
+            content={"detail": errors},
+        )
+
     @app.get("/api/health", tags=["meta"])
     def health() -> dict[str, str]:
         """Report basic liveness so the UI can confirm the server is reachable."""