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/sqlc/queries/databases.sql

@@ -1,16 +1,16 @@
 -- name: ListDatabaseServicesByUser :many
-SELECT id, user_id, name, type, status, version, plan, region, connection_url, backup_schedule, next_backup_at, created_at, updated_at
+SELECT database_services.*
 FROM database_services
 WHERE user_id = $1
 ORDER BY created_at DESC;
 
 -- name: ListAllDatabaseServices :many
-SELECT id, user_id, name, type, status, version, plan, region, connection_url, backup_schedule, next_backup_at, created_at, updated_at
+SELECT database_services.*
 FROM database_services
 ORDER BY created_at DESC;
 
 -- name: GetDatabaseServiceByID :one
-SELECT id, user_id, name, type, status, version, plan, region, connection_url, backup_schedule, next_backup_at, created_at, updated_at
+SELECT database_services.*
 FROM database_services
 WHERE id = $1;
 
@@ -20,7 +20,7 @@ FROM database_services
 WHERE id = $1;
 
 -- name: GetDatabaseServiceByIDAndUser :one
-SELECT id, user_id, name, type, status, version, plan, region, connection_url, backup_schedule, next_backup_at, created_at, updated_at
+SELECT database_services.*
 FROM database_services
 WHERE id = $1 AND user_id = $2;
 
@@ -110,3 +110,9 @@ WHERE backup_schedule IS NOT NULL AND backup_schedule <> ''
 UPDATE database_services
 SET next_backup_at = $1
 WHERE id = $2;
+
+-- name: CreateExternalDatabaseService :exec
+INSERT INTO database_services (id, user_id, name, type, status, version, plan, region,
+    provider, external_host, external_port, external_name, external_username, external_password, external_ssl,
+    created_at, updated_at)
+VALUES ($1, $2, $3, $4, 'running', $5, $6, $7, 'external', $8, $9, $10, $11, $12, $13, $14, $15);