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/web/tracker/infrastructure/src/lib/graphql/fragments.graphql

@@ -51,6 +51,13 @@ fragment ProjectFields on Project {
       name
       archived
     }
+    customFields {
+      id
+      name
+      type
+      options
+      archived
+    }
   }
 }
 
@@ -95,6 +102,10 @@ fragment IssueSummaryFields on Issue {
   storyPoints
   startDate
   dueDate
+  customFields {
+    fieldId
+    value
+  }
   trashedAt
 }