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)

openapi/openapi.json

@@ -350,6 +350,76 @@
         }
       }
     },
+    "/api/config/smtp/test": {
+      "post": {
+        "tags": [
+          "api-config"
+        ],
+        "summary": "Post Smtp Test",
+        "description": "Send a test SMTP email using the current runtime settings.\n\nReturns a structured result indicating success or the category of failure.\nThree possible outcomes:\n- 200 { \"result\": \"success\", \"message\": ... }\n- 400 { \"result\": \"config-error\", \"message\": ... }   (EmailConfigurationError)\n- 502 { \"result\": \"failed\", \"message\": ... }          (EmailDeliveryError)\n\nSMTP credentials are never echoed in the response.",
+        "operationId": "post_smtp_test_api_config_smtp_test_post",
+        "parameters": [
+          {
+            "name": "X-CSRF-Token",
+            "in": "header",
+            "required": false,
+            "schema": {
+              "anyOf": [
+                {
+                  "type": "string"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "title": "X-Csrf-Token"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Successful Response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/SmtpTestResponse"
+                }
+              }
+            }
+          },
+          "400": {
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/SmtpTestResponse"
+                }
+              }
+            },
+            "description": "Bad Request"
+          },
+          "502": {
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/SmtpTestResponse"
+                }
+              }
+            },
+            "description": "Bad Gateway"
+          },
+          "422": {
+            "description": "Validation Error",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HTTPValidationError"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/locations": {
       "get": {
         "tags": [
@@ -1749,6 +1819,30 @@
         ],
         "title": "SessionUser"
       },
+      "SmtpTestResponse": {
+        "properties": {
+          "result": {
+            "type": "string",
+            "enum": [
+              "success",
+              "config-error",
+              "failed"
+            ],
+            "title": "Result"
+          },
+          "message": {
+            "type": "string",
+            "title": "Message"
+          }
+        },
+        "type": "object",
+        "required": [
+          "result",
+          "message"
+        ],
+        "title": "SmtpTestResponse",
+        "description": "Response from POST /api/config/smtp/test."
+      },
       "StatusResponse": {
         "properties": {
           "status": {