feat(schedulerclient): WatchJobs SSE stream + fix ListJobs filter param

Add WatchJobs(ctx) to the Client interface and HTTPClient: opens the
apiserver's /xce/v1/events SSE stream and yields JobEvent values until the
context is cancelled or the stream ends. JobEvent carries the event type,
job id, status string, exit code, and a raw job snapshot.

Also fix ListJobs' filter params: the apiserver accepts filters via the
"filter" query param in field:op:value form (e.g. queue_name:eq:normal);
the individual queue_name/submit_user params it ignores with a 400, which
broke every filtered list. Multiple conditions are brace-wrapped {a},{b}.

Verified live on t1: WatchJobs captured job.updated PEND→RUN→DONE for a
submitted job.

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

src/frontend/schedulerclient/client.go

@@ -50,4 +50,12 @@ type Client interface {
 
 	// ReleaseJob resumes a suspended job (action "resume", default SIGCONT).
 	ReleaseJob(ctx context.Context, id string) error
+
+	// ── Event stream (Phase 2) ──
+
+	// WatchJobs subscribes to the live job-event SSE stream and returns a
+	// channel of lifecycle events (job.created / job.updated / job.deleted /
+	// job.finished). The channel is closed when the context is cancelled or
+	// the stream ends. The caller is responsible for cancelling ctx.
+	WatchJobs(ctx context.Context) (<-chan JobEvent, error)
 }