docs: add repo layout structure map

docs/structure-map.md

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+# LegacyHUB structure map
+
+Status: current mapping · 2026-07-06.
+
+Canonical target: `C:\Users\manag\TeamHUB\TeamHUB-Platform\docs\submodules\18_REPO_LAYOUT_STANDARD.md`.
+
+## Current layout
+
+```text
+legacy-knowledge-indexer/
+  AGENTS.md
+  README.md
+  RUNBOOK.md
+  app/
+    main.py
+    config.py
+    api/
+    common/
+    db/
+    indexing/
+    ingestion/
+    integrations/
+    storage/
+    utils/
+    workers/
+  frontend/src/
+  scripts/
+  tests/
+  docker/
+  docs/
+```
+
+## Target mapping
+
+| Current path | Target layer | Notes |
+|---|---|---|
+| `app/main.py` | backend composition shell | Current mature runtime shell. |
+| `app/api/` | `backend/app/api/` | Route and schema layer. |
+| `app/db/` | `backend/app/models` + `repositories` | Current DB/session/models/migrations boundary. |
+| `app/ingestion/` | services/use cases | OCR/extract/chunk pipeline; do not rename without extraction plan. |
+| `app/indexing/` | services/integrations | Search/vector/rerank runtime; shared parts move through Engines. |
+| `app/storage/` | integrations/storage + repositories | MinIO and artifact persistence. |
+| `app/integrations/` | integrations | Identity/dispatch/reference adapters should live here. |
+| `app/workers/` | workers | Celery tasks. |
+| `frontend/src/` | frontend target | React/Vite/Tailwind app. |
+
+## Compatibility surfaces
+
+- Root `app/` is accepted current backend root.
+- `ingestion`, `indexing` and `storage` are mature domain layers; do not flatten
+  or rename only for symmetry.
+- Shared OCR/document-recognition logic should move through `Engines`, leaving
+  thin app adapters.
+
+## Next safe steps
+
+1. Add new platform adapters in `app/integrations`.
+2. Document current layer mapping instead of moving ingestion/search.
+3. Use focused pytest/compileall and frontend build checks for touched runtime.