chore: migrate clustermode/examples to use generic pull queue

[nahapetyan-serob]

This PR migrates the examples/clustermode server from its DB-backed event queue to the generic pull event queue introduced in #354.
New puller.go: implements eventqueue.Puller
Deleted eventqueue.go: replaced by the generic pull manager.
client/main.go: on SSE stream error during send, fall through to subscribe(taskID) so the client resumes after transient drops.
Dockerfile.app: bump Go 1.24 → 1.25 and adjust build paths for the example's module layout.

The puller tolerates ErrTaskNotFound on the initial snapshot and returns a synthetic TASK_STATE_SUBMITTED placeholder, handling the race where the frontend creates the reader before the backend worker has inserted the initial task row. `

[gemini-code-assist]

Code Review

This pull request replaces the custom database event queue manager with a pull-based queue manager using a new dbPuller implementation in the cluster mode example. It also updates the Dockerfile to Go 1.25, improves error handling in the client streaming logic, and ensures optimistic concurrency failures wrap taskstore.ErrConcurrentModification. The review feedback suggests several improvements: ignoring stream errors if a terminal event has already been received, returning an error instead of silently continuing if scanning database rows fails, and checking rows.Err() after iterating through database rows to prevent premature termination issues.

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