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/migrations/00003_db_connections.sql

@@ -0,0 +1,18 @@
+-- +goose Up
+CREATE TABLE core.db_connections (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  project_id UUID NOT NULL REFERENCES core.projects(id) ON DELETE CASCADE,
+  name TEXT NOT NULL,
+  db_type TEXT NOT NULL,
+  config JSONB NOT NULL DEFAULT '{}'::jsonb,
+  is_managed BOOLEAN NOT NULL DEFAULT FALSE,
+  created_by_user_id UUID REFERENCES core.users(id) ON DELETE SET NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  UNIQUE (project_id, name)
+);
+
+CREATE INDEX idx_db_connections_project_id ON core.db_connections(project_id);
+
+-- +goose Down
+DROP TABLE IF EXISTS core.db_connections;