perf(actions): debounce runner heartbeat writes and throttle task picks (#38281)

3 reductions in the DB load generated by many runners polling
`FetchTask`:

**1. Debounce runner heartbeat writes**
Every poll wrote `last_online`, and every `UpdateTask`/`UpdateLog` wrote
`last_active` — while a runner streams logs that is many writes per
second per runner. These are now persisted only when stale enough to
actually affect the active/offline status (`ShouldPersistLastOnline` /
`ShouldPersistLastActive`), using the existing columns.

**2. Throttle concurrent task picks**
A new in-process semaphore (`MAX_CONCURRENT_TASK_PICKS`) bounds how many
runners run the task-assignment transaction at once, so a fleet polling
together cannot stampede the query. Throttled polls retry on their next
poll without advancing the runner's tasks version.

**3. Paginate the task-pick query**
`CreateTaskForRunner` previously loaded every waiting job in the
runner's scope into memory on each poll (no `LIMIT`). Now it pages
through the waiting backlog oldest-first with `LIMIT`, claiming the
first label-matching job.

---------

Co-authored-by: Zettat123 <zettat123@gmail.com>

models/actions/runner_test.go

@@ -6,14 +6,86 @@ package actions
 import (
 	"fmt"
 	"testing"
+	"time"
 
 	"gitea.dev/models/db"
 	"gitea.dev/models/unittest"
+	"gitea.dev/modules/timeutil"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
+func TestShouldPersistLastOnline(t *testing.T) {
+	now := time.Now()
+	tests := []struct {
+		name string
+		last timeutil.TimeStamp
+		want bool
+	}{
+		{
+			name: "fresh, skip write",
+			last: timeutil.TimeStamp(now.Add(-5 * time.Second).Unix()),
+			want: false,
+		},
+		{
+			name: "exactly at interval, write",
+			last: timeutil.TimeStamp(now.Add(-RunnerHeartbeatInterval).Unix()),
+			want: true,
+		},
+		{
+			name: "stale, write",
+			last: timeutil.TimeStamp(now.Add(-2 * RunnerHeartbeatInterval).Unix()),
+			want: true,
+		},
+		{
+			name: "zero (never seen), write",
+			last: 0,
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, ShouldPersistLastOnline(tt.last, now))
+		})
+	}
+}
+
+func TestShouldPersistLastActive(t *testing.T) {
+	now := time.Now()
+	tests := []struct {
+		name string
+		last timeutil.TimeStamp
+		want bool
+	}{
+		{
+			name: "fresh, skip write",
+			last: timeutil.TimeStamp(now.Add(-1 * time.Second).Unix()),
+			want: false,
+		},
+		{
+			name: "exactly at interval, write",
+			last: timeutil.TimeStamp(now.Add(-RunnerActiveInterval).Unix()),
+			want: true,
+		},
+		{
+			name: "stale, write",
+			last: timeutil.TimeStamp(now.Add(-2 * RunnerActiveInterval).Unix()),
+			want: true,
+		},
+		{
+			name: "zero (never seen), write",
+			last: 0,
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, ShouldPersistLastActive(tt.last, now))
+		})
+	}
+}
+
 func TestFindRunnerOptions_ToOrders_StableTiebreaker(t *testing.T) {
 	// Sorts on a non-unique column must end with the unique id tiebreaker so
 	// pagination is deterministic; without it, runners sharing the same