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/config/config.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 )
@@ -14,6 +15,7 @@ type Config struct {
 	DatabaseURL           string
 	DragonflyURL          string
 	StorageRoot           string
+	DBXDataDir            string
 	AuthInternalBaseURL   string
 	PublicURL             string
 	UserRateLimitPerMin   int
@@ -40,6 +42,7 @@ func Load() (Config, error) {
 		DatabaseURL:           os.Getenv("DATABASE_URL"),
 		DragonflyURL:          getenv("DRAGONFLY_URL", "redis://localhost:6379/0"),
 		StorageRoot:           getenv("BACKEND_STORAGE_ROOT", "./tmp/storage"),
+		DBXDataDir:            os.Getenv("DBX_DATA_DIR"),
 		AuthInternalBaseURL:   getenv("AUTH_INTERNAL_BASE_URL", "http://auth:3001"),
 		PublicURL:             getenv("VITE_APP_URL", "http://localhost"),
 		UserRateLimitPerMin:   240,
@@ -104,6 +107,10 @@ func Load() (Config, error) {
 		return Config{}, errors.New("missing required environment values: " + strings.Join(missing, ", "))
 	}
 
+	if cfg.DBXDataDir == "" {
+		cfg.DBXDataDir = filepath.Join(cfg.StorageRoot, "dbx")
+	}
+
 	return cfg, nil
 }