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/db/queries/db_connections.sql

@@ -0,0 +1,30 @@
+-- name: ListDBConnections :many
+SELECT * FROM core.db_connections
+WHERE project_id = $1
+ORDER BY is_managed DESC, name ASC;
+
+-- name: GetDBConnectionByID :one
+SELECT * FROM core.db_connections
+WHERE id = $1;
+
+-- name: GetDBConnectionByName :one
+SELECT * FROM core.db_connections
+WHERE project_id = $1
+  AND name = $2;
+
+-- name: CreateDBConnection :one
+INSERT INTO core.db_connections (
+  project_id,
+  name,
+  db_type,
+  config,
+  is_managed,
+  created_by_user_id
+) VALUES ($1, $2, $3, $4, $5, $6)
+RETURNING *;
+
+-- name: DeleteDBConnection :one
+DELETE FROM core.db_connections
+WHERE id = $1
+  AND project_id = $2
+RETURNING *;