feat: document delete permissions + three-dots menu portal fix

- Add can_delete column to document_shares (migration 0005)
- Inject x-user-is-admin header from backend proxy to doc-service
- Add get_user_is_admin() dep in doc-service
- Delete endpoint now allows: owner, admin, or group member with can_delete=true
- Watch documents (user_id='watch') deletable by admins only
- DocumentOut gains viewer_can_delete (computed per-request)
- Share UI: 'Allow group members to delete' checkbox + trash badge on shares
- RowActionsMenu dropdown portaled to document.body — fixes overflow-hidden clipping
- Delete mutation onError handler — no more silent failures

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

changelog/2026-04-18_document-delete-permissions.md

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+# 2026-04-18 — Document delete permissions + three-dots menu fix
+
+**Timestamp:** 2026-04-18T00:00:00Z
+
+## Summary
+
+Added a proper permission model for document deletion: owners and admins can always delete; group members can delete only when the share was explicitly granted `can_delete=true`. Fixed silent delete failures (watch docs returning 404 with no user feedback) and fixed the three-dots context menu being clipped by `overflow-hidden` on the table container.
+
+## Files Added / Modified / Deleted
+
+### Added
+- `features/doc-service/alembic/versions/0005_add_share_can_delete.py` — migration: adds `can_delete BOOLEAN NOT NULL DEFAULT false` to `document_shares`
+
+### Modified
+- `features/doc-service/app/models/document_share.py` — added `can_delete: Mapped[bool]` column
+- `features/doc-service/app/schemas/share.py` — added `can_delete` to `DocumentShareOut` and `DocumentShareCreate`; added `viewer_can_delete` to `SharedDocumentOut`
+- `features/doc-service/app/schemas/document.py` — added `viewer_can_delete: bool = False` to `DocumentOut`
+- `features/doc-service/app/deps.py` — added `get_user_is_admin()` dep reading `x-user-is-admin` header
+- `features/doc-service/app/routers/documents.py` — added `_get_deletable_doc_ids()` helper; updated list/get/delete endpoints with permission logic; updated `add_share` to store `can_delete`; updated shared-with-me to include `viewer_can_delete`
+- `backend/app/routers/documents_proxy.py` — `_forward_headers()` now injects `x-user-is-admin` header
+- `frontend/src/api/client.ts` — `DocumentOut`: added `viewer_can_delete`; `DocumentShareOut`: added `can_delete`; `addDocumentShare`: accepts `canDelete` param
+- `frontend/src/pages/DocumentsPage.tsx` — `RowActionsMenu`: replaced absolute dropdown with `createPortal` to fix clipping; delete button now uses `doc.viewer_can_delete`; added `onError` handler for silent failures
+- `frontend/src/components/DocumentSlideOver.tsx` — sharing section: shows trash icon badge on shares with `can_delete=true`; added "Allow group members to delete" checkbox before group picker; delete button uses `doc.viewer_can_delete`
+- `features/doc-service/CLAUDE.md` — updated `document_shares` table docs + migration chain
+- `backend/CLAUDE.md` — noted `x-user-is-admin` header injection