refactor: add table primitive and migrations by date; feat: add mcp connectors

backend/migrations/20260610_01_soft_deleted_document_versions.sql

@@ -0,0 +1,16 @@
+-- Migration date: 2026-06-10
+
+-- Keep document version tombstones after deleting version file bytes.
+-- Deleted versions remain visible in history but are ignored by active-file
+-- lookups and cannot be opened/downloaded/replaced.
+
+alter table public.document_versions
+  alter column storage_path drop not null;
+
+alter table public.document_versions
+  add column if not exists deleted_at timestamptz,
+  add column if not exists deleted_by uuid;
+
+create index if not exists document_versions_active_document_id_idx
+  on public.document_versions(document_id, created_at desc)
+  where deleted_at is null;