feat: custom fields per project

Text, number, date or option-list fields defined in the project settings
(archived, never removed; their type is fixed once saved). Tickets hold
them, edited in the side panel, listed under the title and filtered by
option in the list view; also over GraphQL, REST (?field=ID:value) and MCP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/presenters/src/lib/common/settings.ts

@@ -1,4 +1,5 @@
 import {
+  CustomFieldType,
   ProjectSettings,
   SettingColor,
   StatusCategory,
@@ -26,11 +27,20 @@ export interface SettingsPayload {
     template?: { description?: string | null; checklist?: string[] } | null;
   }[];
   areas: { id: string; name: string; archived: boolean }[];
+  /** Absent when saving: unchanged. */
+  customFields?: {
+    id: string;
+    name: string;
+    type: string;
+    options?: string[];
+    archived: boolean;
+  }[];
 }
 
 export function toSettingsPayload(settings: ProjectSettings): SettingsPayload {
   return {
     ...settings,
+    customFields: settings.customFields ?? [],
     transitions: settings.statuses.map((s) => ({
       from: s.id,
       to: settings.transitions[s.id] ?? [],
@@ -62,5 +72,14 @@ export function fromSettingsPayload(payload: SettingsPayload): ProjectSettings {
         : {}),
     })),
     areas: payload.areas,
+    ...(payload.customFields
+      ? {
+          customFields: payload.customFields.map((f) => ({
+            ...f,
+            type: f.type as CustomFieldType,
+            options: f.options ?? [],
+          })),
+        }
+      : {}),
   };
 }