feat(stamdata): admin stamdata maintenance editor (beheer)

Realizes ADR-0004's "future low-code editor that commits a PR": an
admin-only stamdata maintenance editor built on the stamdata-as-code
foundation.

Backend: `professions` moves from a hardcoded C# dictionary to an embedded
`professions.json` data-file (typed as `ProfessionMapping`) with valid-time
(geldigVan/geldigTot, half-open). A generic, reflection-driven
StamdataCatalog/StamdataTable/StamdataFile describes every table so one
endpoint pair + one grid editor serve all of them; add a table in one line.
Two read-only, admin-gated endpoints (GET /stamdata, GET /stamdata/{table}
?peildatum=) — no runtime write path. Generic build gate
`Every_catalog_table_is_valid` (keys non-blank, no overlapping validity,
well-formed windows).

Frontend: new `beheer` context (route beheer/stamdata, capabilityGuard
'stamdata:edit'). A schema-driven grid editor edits rows locally; download()
emits {table}.json for the admin to commit as a reviewed PR (no mutation
command — the CI build + StamdataValidationTests stay the authority).

Full gate GREEN both sides; gen:api leaves no drift; new stamdata story
passes axe. See WP-29 + ADR-0004.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/swagger.json

@@ -127,6 +127,89 @@
         }
       }
     },
+    "/api/v1/stamdata": {
+      "get": {
+        "tags": [
+          "BigRegister.Api, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
+        ],
+        "operationId": "stamdataTables",
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/StamdataTableSummaryDto"
+                  }
+                }
+              }
+            }
+          },
+          "403": {
+            "description": "Forbidden",
+            "content": {
+              "application/problem+json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
+    "/api/v1/stamdata/{table}": {
+      "get": {
+        "tags": [
+          "BigRegister.Api, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
+        ],
+        "operationId": "stamdataTable",
+        "parameters": [
+          {
+            "name": "table",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          },
+          {
+            "name": "peildatum",
+            "in": "query",
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/StamdataTableDto"
+                }
+              }
+            }
+          },
+          "403": {
+            "description": "Forbidden",
+            "content": {
+              "application/problem+json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          },
+          "404": {
+            "description": "Not Found"
+          }
+        }
+      }
+    },
     "/api/v1/registrations": {
       "post": {
         "tags": [
@@ -2178,6 +2261,83 @@
         },
         "additionalProperties": false
       },
+      "StamdataColumnDto": {
+        "type": "object",
+        "properties": {
+          "name": {
+            "type": "string",
+            "nullable": true
+          },
+          "type": {
+            "type": "string",
+            "nullable": true
+          },
+          "isKey": {
+            "type": "boolean"
+          },
+          "options": {
+            "type": "array",
+            "items": {
+              "type": "string"
+            },
+            "nullable": true
+          }
+        },
+        "additionalProperties": false
+      },
+      "StamdataTableDto": {
+        "type": "object",
+        "properties": {
+          "id": {
+            "type": "string",
+            "nullable": true
+          },
+          "label": {
+            "type": "string",
+            "nullable": true
+          },
+          "columns": {
+            "type": "array",
+            "items": {
+              "$ref": "#/components/schemas/StamdataColumnDto"
+            },
+            "nullable": true
+          },
+          "temporal": {
+            "type": "boolean"
+          },
+          "rows": {
+            "type": "array",
+            "items": { },
+            "nullable": true
+          }
+        },
+        "additionalProperties": false
+      },
+      "StamdataTableSummaryDto": {
+        "type": "object",
+        "properties": {
+          "id": {
+            "type": "string",
+            "nullable": true
+          },
+          "label": {
+            "type": "string",
+            "nullable": true
+          },
+          "columns": {
+            "type": "array",
+            "items": {
+              "$ref": "#/components/schemas/StamdataColumnDto"
+            },
+            "nullable": true
+          },
+          "temporal": {
+            "type": "boolean"
+          }
+        },
+        "additionalProperties": false
+      },
       "SubOrgSummaryDto": {
         "type": "object",
         "properties": {