feat(deploy): webhook queue+nocache, restart sweep, secrets at rest

- Webhook-triggered deploys now go through the per-service deploy queue
  and default to no_cache builds (dflow convention — prevents stale
  layer surprises on push).
- Boot sweep marks deployments interrupted by restart as failed instead
  of leaving phantom queued/building rows.
- is_secret environment variables are AES-GCM-encrypted at rest
  (enc:v1: prefix); key from SECRETS_KEY or JWT_SECRET fallback.
  Existing plaintext secrets encrypt on boot; deploy/exec paths decrypt
  transparently. Masked-echo writes preserve stored ciphertext.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/secrets/secrets_test.go

@@ -0,0 +1,63 @@
+package secrets
+
+import (
+	"sync"
+	"testing"
+)
+
+// resetForTest restores the package state so Configure can be re-run.
+func resetForTest() {
+	keyOnce = sync.Once{}
+	key = nil
+	enabled = false
+}
+
+func TestEncryptDecryptRoundTrip(t *testing.T) {
+	resetForTest()
+	Configure("test-secrets-key", "")
+
+	enc := Encrypt("super-secret-value")
+	if !IsEncrypted(enc) {
+		t.Fatalf("expected encrypted prefix, got %q", enc)
+	}
+	if enc == "super-secret-value" {
+		t.Fatal("value stored as plaintext")
+	}
+	if got := Decrypt(enc); got != "super-secret-value" {
+		t.Fatalf("decrypt = %q", got)
+	}
+}
+
+func TestPlaintextPassThrough(t *testing.T) {
+	resetForTest()
+	Configure("test-secrets-key", "")
+	if got := Decrypt("legacy-plaintext"); got != "legacy-plaintext" {
+		t.Fatalf("legacy plaintext changed: %q", got)
+	}
+}
+
+func TestEncryptIsIdempotent(t *testing.T) {
+	resetForTest()
+	Configure("test-secrets-key", "")
+	enc := Encrypt("v")
+	if got := Encrypt(enc); got != enc {
+		t.Fatal("double-encrypt changed the value")
+	}
+}
+
+func TestDecryptFailsSoftOnCorruption(t *testing.T) {
+	resetForTest()
+	Configure("test-secrets-key", "")
+	corrupt := Prefix + "not-valid-base64!!!"
+	if got := Decrypt(corrupt); got != corrupt {
+		t.Fatal("corrupt ciphertext should pass through unchanged")
+	}
+}
+
+func TestDisabledWithoutKey(t *testing.T) {
+	resetForTest()
+	Configure("", "")
+	if got := Encrypt("v"); got != "v" {
+		t.Fatal("encrypt should pass through when no key configured")
+	}
+}