feat: external databases — register, probe, encrypted credentials

Databases hosted outside Containr can now be registered alongside
managed ones. The connection is probed before storing (protocol-level
for postgres/redis-family via real drivers, TCP dial for the rest),
credentials are encrypted at rest and never returned, and external rows
skip managed lifecycle actions, backups, and Docker metrics.

- POST /databases/register-external — probe + create
- POST /databases/test-connection — ad-hoc probe (ok/latency_ms/error)
- POST /databases/{id}/test-connection — re-probe stored credentials
- provider/external fields on database services; privileged reads get a
  composed connection URL with loopback hosts rewritten to the caller's
- CLI: databases register + test-connection; MCP: 3 new tools
- UI: managed/external toggle in the create modal, external badge and
  host display, per-row test button, backups section hidden
- Migration widens the plan CHECK to admit 'external'

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

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

app/backend/internal/api/dbexternal_test.go

@@ -0,0 +1,97 @@
+package api
+
+import (
+	"context"
+	"net"
+	"testing"
+)
+
+func TestNormalizeExternalRequestDefaults(t *testing.T) {
+	req := externalDatabaseRequest{Type: "PostgreSQL", Host: " db.example.com ", Port: 0}
+	if err := normalizeExternalRequest(&req); err != nil {
+		t.Fatalf("normalize failed: %v", err)
+	}
+	if req.Type != "postgresql" {
+		t.Fatalf("type not normalized: %q", req.Type)
+	}
+	if req.Host != "db.example.com" {
+		t.Fatalf("host not trimmed: %q", req.Host)
+	}
+	if req.Port != 5432 {
+		t.Fatalf("default port = %d, want 5432", req.Port)
+	}
+}
+
+func TestNormalizeExternalRequestRejects(t *testing.T) {
+	cases := []externalDatabaseRequest{
+		{Type: "sqlite", Host: "x"},                  // unsupported type
+		{Type: "postgresql", Host: "  "},             // empty host
+		{Type: "postgresql", Host: "h", Port: 70000}, // out of range
+	}
+	for i, req := range cases {
+		if err := normalizeExternalRequest(&req); err == nil {
+			t.Fatalf("case %d: expected error", i)
+		}
+	}
+}
+
+func TestExternalConnectionURLMasking(t *testing.T) {
+	ext := &DatabaseExternalRef{
+		Host:     "db.example.com",
+		Port:     5432,
+		Database: "appdb",
+		Username: "svc",
+		SSL:      true,
+	}
+	u := externalConnectionURL("postgresql", ext, "s3cret")
+	if u != "postgres://svc:[email protected]:5432/appdb?sslmode=require" {
+		t.Fatalf("unexpected pg url: %s", u)
+	}
+	u = externalConnectionURL("postgresql", ext, "")
+	if u != "postgres://[email protected]:5432/appdb?sslmode=require" {
+		t.Fatalf("unexpected passwordless url: %s", u)
+	}
+
+	r := &DatabaseExternalRef{Host: "cache.internal", Port: 6379, SSL: true}
+	u = externalConnectionURL("redis", r, "pw")
+	if u != "rediss://:[email protected]:6379" {
+		t.Fatalf("unexpected redis url: %s", u)
+	}
+}
+
+func TestProbeExternalDatabaseTCP(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	defer ln.Close()
+	port := ln.Addr().(*net.TCPAddr).Port
+
+	// TCP fallback path — mysql uses a plain dial.
+	latency, err := probeExternalDatabase(context.Background(), externalDatabaseRequest{
+		Type: "mysql", Host: "127.0.0.1", Port: port,
+	})
+	if err != nil {
+		t.Fatalf("probe should succeed against open listener: %v", err)
+	}
+	if latency <= 0 {
+		t.Fatalf("latency not recorded")
+	}
+
+	// Closed port must fail fast.
+	if _, err := probeExternalDatabase(context.Background(), externalDatabaseRequest{
+		Type: "mysql", Host: "127.0.0.1", Port: 1,
+	}); err == nil {
+		t.Fatalf("probe should fail against closed port")
+	}
+}
+
+func TestRedisTLSConfig(t *testing.T) {
+	if redisTLSConfig(false, "h") != nil {
+		t.Fatalf("disabled ssl must produce nil tls config")
+	}
+	cfg := redisTLSConfig(true, "db.example.com")
+	if cfg == nil || cfg.ServerName != "db.example.com" {
+		t.Fatalf("expected ServerName passthrough, got %+v", cfg)
+	}
+}