M2-T05: add SMTP test action API (POST /api/config/smtp/test)

- reuses send_smtp_test_email; tri-state result success(200)/config-error(400)/failed(502)
- session + CSRF protected; never echoes SMTP secrets
- SmtpTestResponse schema; regenerate openapi/
- extend tests/test_api_config.py (3 states + 401 + missing-CSRF 403)

app/api/routes/api/config.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import logging
 
 from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi.responses import JSONResponse
 from sqlalchemy.orm import Session
 
 from app.api.routes.api.deps import require_csrf, require_session
@@ -14,9 +15,11 @@ from app.schemas.config import (
     ConfigSection,
     ConfigUpdateRequest,
     ConfigUpdateResponse,
+    SmtpTestResponse,
 )
 from app.services.auth import AuthenticatedSession
 from app.services.config_page import ConfigSaveError, build_config_sections, save_config_updates
+from app.services.email import EmailConfigurationError, EmailDeliveryError, send_smtp_test_email
 
 logger = logging.getLogger(__name__)
 
@@ -69,3 +72,48 @@ def put_config(
     refreshed_settings = get_settings()
     sections_raw = build_config_sections(db, refreshed_settings)
     return ConfigUpdateResponse(sections=_sections_from_raw(sections_raw))
+
+
+@router.post(
+    "/config/smtp/test",
+    responses={
+        200: {"model": SmtpTestResponse},
+        400: {"model": SmtpTestResponse},
+        502: {"model": SmtpTestResponse},
+    },
+)
+def post_smtp_test(
+    settings: Settings = Depends(get_app_settings),
+    _auth: AuthenticatedSession = Depends(require_session),
+    _csrf: None = Depends(require_csrf),
+) -> JSONResponse:
+    """
+    Send a test SMTP email using the current runtime settings.
+
+    Returns a structured result indicating success or the category of failure.
+    Three possible outcomes:
+    - 200 { "result": "success", "message": ... }
+    - 400 { "result": "config-error", "message": ... }   (EmailConfigurationError)
+    - 502 { "result": "failed", "message": ... }          (EmailDeliveryError)
+
+    SMTP credentials are never echoed in the response.
+    """
+    try:
+        send_smtp_test_email(settings)
+    except EmailConfigurationError as exc:
+        logger.warning("SMTP test rejected due to configuration: %s", exc)
+        return JSONResponse(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            content={"result": "config-error", "message": str(exc)},
+        )
+    except EmailDeliveryError as exc:
+        logger.warning("SMTP test delivery failed: %s", exc)
+        return JSONResponse(
+            status_code=status.HTTP_502_BAD_GATEWAY,
+            content={"result": "failed", "message": str(exc)},
+        )
+
+    return JSONResponse(
+        status_code=status.HTTP_200_OK,
+        content={"result": "success", "message": "Test email sent successfully."},
+    )