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/schemas/config.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from typing import Literal
+
 from pydantic import BaseModel
 
 
@@ -29,3 +31,10 @@ class ConfigUpdateRequest(BaseModel):
 
 class ConfigUpdateResponse(BaseModel):
     sections: list[ConfigSection]
+
+
+class SmtpTestResponse(BaseModel):
+    """Response from POST /api/config/smtp/test."""
+
+    result: Literal["success", "config-error", "failed"]
+    message: str