fix: cancel in-progress deploy when webhook triggers new deploy (closes #38)

When a webhook-triggered deploy starts for an app that already has a deploy
in progress, the existing deploy is now cancelled via context cancellation
before the new deploy begins. This prevents silently lost webhook deploys.

Changes:
- Add per-app active deploy tracking with cancel func and done channel
- Deploy() accepts cancelExisting param: true for webhook, false for manual
- Cancelled deployments are marked with new 'cancelled' status
- Add ErrDeployCancelled sentinel error
- Add DeploymentStatusCancelled model constant
- Add comprehensive tests for cancellation mechanics

internal/service/deploy/deploy_cancel_test.go

@@ -0,0 +1,133 @@
+package deploy_test
+
+import (
+	"context"
+	"log/slog"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"git.eeqj.de/sneak/upaas/internal/service/deploy"
+)
+
+func TestCancelActiveDeploy_NoExisting(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+
+	// Should not panic or block when no active deploy exists
+	svc.CancelActiveDeploy("nonexistent-app")
+}
+
+func TestCancelActiveDeploy_CancelsAndWaits(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+
+	svc.RegisterActiveDeploy("app-1", cancel, done)
+
+	// Simulate a running deploy that respects cancellation
+	var deployFinished bool
+
+	go func() {
+		<-ctx.Done()
+
+		deployFinished = true
+
+		close(done)
+	}()
+
+	svc.CancelActiveDeploy("app-1")
+	assert.True(t, deployFinished, "deploy should have finished after cancellation")
+}
+
+func TestCancelActiveDeploy_BlocksUntilDone(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+
+	svc.RegisterActiveDeploy("app-2", cancel, done)
+
+	// Simulate slow cleanup after cancellation
+	go func() {
+		<-ctx.Done()
+		time.Sleep(50 * time.Millisecond)
+		close(done)
+	}()
+
+	start := time.Now()
+
+	svc.CancelActiveDeploy("app-2")
+
+	elapsed := time.Since(start)
+
+	assert.GreaterOrEqual(t, elapsed, 50*time.Millisecond,
+		"cancelActiveDeploy should block until the deploy finishes")
+}
+
+func TestTryLockApp_PreventsConcurrent(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+
+	assert.True(t, svc.TryLockApp("app-1"), "first lock should succeed")
+	assert.False(t, svc.TryLockApp("app-1"), "second lock should fail")
+
+	svc.UnlockApp("app-1")
+
+	assert.True(t, svc.TryLockApp("app-1"), "lock after unlock should succeed")
+
+	svc.UnlockApp("app-1")
+}
+
+func TestCancelActiveDeploy_AllowsNewDeploy(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+
+	// Simulate an active deploy holding the lock
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+
+	svc.RegisterActiveDeploy("app-3", cancel, done)
+
+	// Lock the app as if a deploy is in progress
+	assert.True(t, svc.TryLockApp("app-3"))
+
+	// Simulate deploy goroutine: release lock on cancellation
+	var mu sync.Mutex
+
+	released := false
+
+	go func() {
+		<-ctx.Done()
+
+		svc.UnlockApp("app-3")
+
+		mu.Lock()
+		released = true
+		mu.Unlock()
+
+		close(done)
+	}()
+
+	// Cancel should cause the old deploy to release its lock
+	svc.CancelActiveDeploy("app-3")
+
+	mu.Lock()
+	assert.True(t, released)
+	mu.Unlock()
+
+	// Now a new deploy should be able to acquire the lock
+	assert.True(t, svc.TryLockApp("app-3"), "should be able to lock after cancellation")
+
+	svc.UnlockApp("app-3")
+}