feat: phase 2 databases via DBX (v0.3.0)

- dbx-mcp persistent stdio subprocess in Go backend (internal/dbx),
  lazy start + single retry, tool failures surface as 502
- core.db_connections registry: per-project connections, credentials
  server-side only, managed entries protected (409)
- platform-postgres + platform-dragonfly auto-register per project
- REST proxy: list/create/delete/test, databases, tables, describe,
  schema context, SQL query, redis command
- Databases page: connection list, schema/table browser, column
  inspector, SQL editor with result grid + row detail, redis console
- backend image moved to debian-slim for glibc; pinned dbx release
  with SHA256 verification

Authored By: TDvorak <[email protected]>

apps/backend/internal/database/db/models.go

@@ -200,6 +200,18 @@ type CoreCollection struct {
 	UpdatedAt       pgtype.Timestamptz `json:"updated_at"`
 }
 
+type CoreDbConnection struct {
+	ID              uuid.UUID          `json:"id"`
+	ProjectID       uuid.UUID          `json:"project_id"`
+	Name            string             `json:"name"`
+	DbType          string             `json:"db_type"`
+	Config          []byte             `json:"config"`
+	IsManaged       bool               `json:"is_managed"`
+	CreatedByUserID pgtype.UUID        `json:"created_by_user_id"`
+	CreatedAt       pgtype.Timestamptz `json:"created_at"`
+	UpdatedAt       pgtype.Timestamptz `json:"updated_at"`
+}
+
 type CoreDocument struct {
 	ID              uuid.UUID          `json:"id"`
 	CollectionID    uuid.UUID          `json:"collection_id"`