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>

features/doc-service/app/models/document_share.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, String, UniqueConstraint, func
+from sqlalchemy import Boolean, DateTime, String, UniqueConstraint, func
 from sqlalchemy.orm import Mapped, mapped_column
 
 from app.database import Base
@@ -14,6 +14,7 @@ class DocumentShare(Base):
     document_id: Mapped[str] = mapped_column(String, nullable=False, index=True)
     group_id: Mapped[str] = mapped_column(String, nullable=False, index=True)
     shared_by_user_id: Mapped[str] = mapped_column(String, nullable=False)
+    can_delete: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), nullable=False
     )