fix(api): retry commands that clash with a concurrent write

Services run commands through a CommandRunner that runs one again (up to
5 times, waiting a little longer each time) when its aggregate changed
underneath, e.g. a saga writing to the same ticket. Each command loads its
aggregate fresh and stores its events in one transaction, so a
ConcurrencyError means nothing was stored and running it again is safe.
People no longer get a 409 for it, and the e2e suite stops flaking.

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

libs/api/tracker/application/src/lib/calendar.service.ts

@@ -1,5 +1,6 @@
+import { CommandRunner } from '@task-stream/api/shared/application';
 import { Injectable } from '@nestjs/common';
-import { CommandBus, QueryBus } from '@nestjs/cqrs';
+import { QueryBus } from '@nestjs/cqrs';
 import {
   CreateCalendarFeedCommand,
   GetCalendarIcsQuery,
@@ -12,7 +13,7 @@ import { CalendarFeed } from './ports/calendar-feeds.repository';
 @Injectable()
 export class CalendarService {
   constructor(
-    private readonly commandBus: CommandBus,
+    private readonly commands: CommandRunner,
     private readonly queryBus: QueryBus,
   ) {}
 
@@ -22,11 +23,11 @@ export class CalendarService {
 
   /** A new secret URL; the previous one, if any, stops working. */
   create(userId: string): Promise<CalendarFeed> {
-    return this.commandBus.execute(new CreateCalendarFeedCommand(userId));
+    return this.commands.execute(new CreateCalendarFeedCommand(userId));
   }
 
   revoke(userId: string): Promise<void> {
-    return this.commandBus.execute(new RevokeCalendarFeedCommand(userId));
+    return this.commands.execute(new RevokeCalendarFeedCommand(userId));
   }
 
   /** The calendar behind `token`, or null if no feed has it. */