feat(admin): runtime feature flags (catalog-in-code, admin toggle, FE+backend)

Catalog declared in code (Domain/Features/FeatureFlags.cs, build-validated), on/off state
persisted in SQLite (FeatureFlagStore + migration). GET /flags (drives FE gating) + admin
PUT /admin/flags/{key} (new flags:manage capability + FlagsAdmin gate). Enforced end-to-end:
the `inschrijving-open` flag hides the Inschrijven nav item + dashboard action (FE) AND makes
POST /applications for a registratie 403 when off (backend). FE FeatureFlagStore mirrors
AccessStore (enabled() deny-by-default); admin toggle page at /beheer/functies in ADMIN_LINKS.
+4 backend tests, /me cap-list updated, client regenerated.

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

backend/swagger.json

@@ -852,6 +852,73 @@
         }
       }
     },
+    "/api/v1/flags": {
+      "get": {
+        "tags": [
+          "BigRegister.Api, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
+        ],
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/FeatureFlagDto"
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
+    "/api/v1/admin/flags/{key}": {
+      "put": {
+        "tags": [
+          "BigRegister.Api, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
+        ],
+        "parameters": [
+          {
+            "name": "key",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/SetFeatureFlagRequest"
+              }
+            }
+          },
+          "required": true
+        },
+        "responses": {
+          "204": {
+            "description": "No Content"
+          },
+          "404": {
+            "description": "Not Found"
+          },
+          "403": {
+            "description": "Forbidden",
+            "content": {
+              "application/problem+json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/brief": {
       "get": {
         "tags": [
@@ -1830,6 +1897,23 @@
         },
         "additionalProperties": false
       },
+      "FeatureFlagDto": {
+        "type": "object",
+        "properties": {
+          "key": {
+            "type": "string",
+            "nullable": true
+          },
+          "description": {
+            "type": "string",
+            "nullable": true
+          },
+          "enabled": {
+            "type": "boolean"
+          }
+        },
+        "additionalProperties": false
+      },
       "HerregistratieDecisionsDto": {
         "type": "object",
         "properties": {
@@ -2396,6 +2480,15 @@
         },
         "additionalProperties": false
       },
+      "SetFeatureFlagRequest": {
+        "type": "object",
+        "properties": {
+          "enabled": {
+            "type": "boolean"
+          }
+        },
+        "additionalProperties": false
+      },
       "StamdataColumnDto": {
         "type": "object",
         "properties": {