feat: release notes

Each release can carry notes in Markdown, written before or after it
ships. A draft is generated from its finished tickets grouped by type, to
edit and copy from the releases page.

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

libs/web/tracker/domain/src/lib/models/models.ts

@@ -206,6 +206,8 @@ export interface Release {
   targetDate: string | null;
   status: ReleaseStatus;
   releasedAt: string | null;
+  /** What shipped, told for people (Markdown). */
+  notes: string | null;
   progress: { total: number; finished: number; percent: number };
 }