feat: allow cancellation of queued deployments

[dennisonbertram]

Problem

When a deployment is triggered (via build or image deploy), there is no way to cancel it if it's still queued or in progress. CI/CD pipelines can trigger mistaken deploys, and the only option is to wait for completion or delete the service entirely.

Proposed Solution

POST /v1/services/{serviceID}/deployments/{deploymentID}/cancel

Cancels a queued or in-progress deployment. Returns 409 if the deployment is already completed or already cancelled.

Response:

{
  "id": "deploy-123",
  "status": "cancelled",
  "cancelled_at": "2026-03-22T10:00:00Z"
}

Behavior

• Queued deployments: Set status to cancelled, remove from queue
• In-progress deployments:
  • If building: cancel the build (send context cancellation)
  • If deploying container: stop the new container, keep the current one running
• Completed/failed/cancelled: Return 409 Conflict

Implementation Notes

• Add handler in internal/api/services.go or internal/api/builds.go
• Use the deployment store's status field
• For in-progress builds, use context cancellation (builds already support this via fix(tenant): cancel active builds during tenant suspension #88)
• Record cancelled_at timestamp and cancelled_by (key ID from auth context)

Acceptance Criteria

• POST /v1/services/{id}/deployments/{id}/cancel cancels queued deployments
• In-progress builds are cancelled via context cancellation
• Completed deployments return 409
• cancelled_at and cancelled_by recorded
• Current running service is not affected by cancellation
• Unit tests for each deployment state transition
• API reference updated

[dennisonbertram]

PR created: #148