feat(config): add license.key configuration option

Add license key configuration under the license section. When empty or
absent, Vikunja runs in community mode with no licensed features.

config-raw.json

@@ -1056,6 +1056,16 @@
                     "comment": "The plugin loader to use. \"yaegi\" loads plugins from Go source files (directories of .go files). \"native\" (deprecated) loads compiled Go plugin shared libraries (.so files)."
                 }
             ]
+        },
+        {
+            "key": "license",
+            "children": [
+                {
+                    "key": "key",
+                    "default_value": "",
+                    "comment": "The license key for Vikunja. If empty or absent, Vikunja runs in community mode with all non-licensed features available."
+                }
+            ]
         }
     ]
 }

pkg/config/config.go

@@ -232,6 +232,8 @@ const (
 	PluginsEnabled Key = `plugins.enabled`
 	PluginsDir     Key = `plugins.dir`
 	PluginsLoader  Key = `plugins.loader`
+
+	LicenseKey Key = `license.key`
 )
 
 var maxFileSizeInBytes uint64
@@ -503,6 +505,8 @@ func InitDefaultConfig() {
 		log.Warningf("Config key %q is deprecated and will be removed in a future release. Please use %q instead.", WebhooksProxyPassword, OutgoingRequestsProxyPassword)
 		OutgoingRequestsProxyPassword.Set(proxyPassword)
 	}
+	// License
+	LicenseKey.setDefault("")
 }
 
 // ResolvePath resolves a path relative to service.rootpath.