feat: Add max-parallel implementation inside the Gitea server

# Conflicts:
#	models/migrations/migrations.go
#	models/migrations/v1_26/v325.go

# Conflicts:
#	models/actions/run_job.go
#	models/actions/runner.go
#	models/migrations/migrations.go
#	modules/structs/repo_actions.go
#	routers/api/actions/runner/runner.go
#	routers/api/v1/admin/runners.go
#	routers/api/v1/api.go
#	routers/api/v1/shared/runners.go
#	services/actions/run.go
#	templates/swagger/v1_json.tmpl

# Conflicts:
#	models/migrations/migrations.go

tests/integration/api_runner_capacity_test.go

@@ -0,0 +1,149 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"testing"
+
+	actions_model "code.gitea.io/gitea/models/actions"
+	auth_model "code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAPIUpdateRunnerCapacity(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	ctx := context.Background()
+
+	// Clean up existing runners
+	require.NoError(t, db.DeleteAllRecords("action_runner"))
+
+	// Create a test runner
+	runner := &actions_model.ActionRunner{
+		UUID:      "test-capacity-runner",
+		Name:      "Test Capacity Runner",
+		OwnerID:   0,
+		RepoID:    0,
+		Capacity:  1,
+		TokenHash: "test-capacity-hash",
+		Token:     "test-capacity-token",
+	}
+	require.NoError(t, actions_model.CreateRunner(ctx, runner))
+
+	// Load the created runner to get the ID
+	runner = unittest.AssertExistsAndLoadBean(t, &actions_model.ActionRunner{UUID: "test-capacity-runner"})
+
+	session := loginUser(t, "user1")
+	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteAdmin)
+
+	t.Run("UpdateCapacity", func(t *testing.T) {
+		req := NewRequestWithJSON(t, "PATCH",
+			fmt.Sprintf("/api/v1/admin/actions/runners/%d/capacity", runner.ID),
+			&api.UpdateRunnerCapacityOption{Capacity: 5}).
+			AddTokenAuth(token)
+
+		resp := MakeRequest(t, req, http.StatusOK)
+
+		var apiRunner api.ActionRunner
+		DecodeJSON(t, resp, &apiRunner)
+
+		assert.Equal(t, runner.ID, apiRunner.ID)
+		assert.Equal(t, 5, apiRunner.Capacity)
+
+		// Verify in database
+		updated, err := actions_model.GetRunnerByID(context.Background(), runner.ID)
+		assert.NoError(t, err)
+		assert.Equal(t, 5, updated.Capacity)
+	})
+
+	t.Run("UpdateCapacityToZero", func(t *testing.T) {
+		req := NewRequestWithJSON(t, "PATCH",
+			fmt.Sprintf("/api/v1/admin/actions/runners/%d/capacity", runner.ID),
+			&api.UpdateRunnerCapacityOption{Capacity: 0}).
+			AddTokenAuth(token)
+
+		resp := MakeRequest(t, req, http.StatusOK)
+
+		var apiRunner api.ActionRunner
+		DecodeJSON(t, resp, &apiRunner)
+
+		assert.Equal(t, 0, apiRunner.Capacity)
+	})
+
+	t.Run("InvalidCapacity", func(t *testing.T) {
+		req := NewRequestWithJSON(t, "PATCH",
+			fmt.Sprintf("/api/v1/admin/actions/runners/%d/capacity", runner.ID),
+			&api.UpdateRunnerCapacityOption{Capacity: -1}).
+			AddTokenAuth(token)
+
+		MakeRequest(t, req, http.StatusBadRequest)
+	})
+
+	t.Run("NonExistentRunner", func(t *testing.T) {
+		req := NewRequestWithJSON(t, "PATCH",
+			"/api/v1/admin/actions/runners/999999/capacity",
+			&api.UpdateRunnerCapacityOption{Capacity: 5}).
+			AddTokenAuth(token)
+
+		MakeRequest(t, req, http.StatusNotFound)
+	})
+
+	t.Run("GetRunnerWithCapacity", func(t *testing.T) {
+		// First set capacity
+		runner.Capacity = 7
+		assert.NoError(t, actions_model.UpdateRunner(context.Background(), runner, "capacity"))
+
+		// Get runner
+		req := NewRequest(t, "GET",
+			fmt.Sprintf("/api/v1/admin/actions/runners/%d", runner.ID)).
+			AddTokenAuth(token)
+
+		resp := MakeRequest(t, req, http.StatusOK)
+
+		var apiRunner api.ActionRunner
+		DecodeJSON(t, resp, &apiRunner)
+
+		assert.Equal(t, runner.ID, apiRunner.ID)
+		assert.Equal(t, 7, apiRunner.Capacity)
+	})
+
+	t.Run("ListRunnersWithCapacity", func(t *testing.T) {
+		req := NewRequest(t, "GET", "/api/v1/admin/actions/runners").
+			AddTokenAuth(token)
+
+		resp := MakeRequest(t, req, http.StatusOK)
+
+		var response api.ActionRunnersResponse
+		DecodeJSON(t, resp, &response)
+
+		// Find our test runner
+		found := false
+		for _, r := range response.Entries {
+			if r.ID == runner.ID {
+				found = true
+				assert.Equal(t, 7, r.Capacity)
+				break
+			}
+		}
+		assert.True(t, found, "Test runner should be in list")
+	})
+
+	t.Run("UnauthorizedAccess", func(t *testing.T) {
+		req := NewRequestWithJSON(t, "PATCH",
+			fmt.Sprintf("/api/v1/admin/actions/runners/%d/capacity", runner.ID),
+			&api.UpdateRunnerCapacityOption{Capacity: 5})
+		// No token
+
+		MakeRequest(t, req, http.StatusUnauthorized)
+	})
+}