docs: adds workspace-invites feature and tasks

docs/TASKS/workspace-invites/001-backend-invite-acceptance-foundation.md

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+# Task: Backend Workspace Invite Foundation
+
+## Goal
+
+Implement the backend data model and endpoints needed to create, list, and accept workspace invites.
+
+## Feature Spec
+
+- `docs/FEATURES/workspace-invites.md`
+
+## Scope
+
+- Add a `WorkspaceInvite` persistence model with workspace id, normalized email, role, status, inviter, token data, timestamps, and expiration.
+- Add invite statuses for `Pending`, `Accepted`, `Cancelled`, and `Expired` without magic strings.
+- Add a manager-only endpoint to create workspace invites.
+- Add a manager-only endpoint to list workspace invites.
+- Prevent duplicate pending invites for the same normalized email in the same workspace.
+- Add a public endpoint to resolve an invite token for display-safe invite details.
+- Add an accept endpoint that validates token, status, expiration, and email match.
+- On acceptance, grant the invited role and `KnownClaims.WorkspaceScope` claim to the user.
+- Mark the invite as accepted in the same transaction as access grants.
+- Add backend tests for create, list, pending, accepted, expired, cancelled, duplicate, and email mismatch paths.
+
+## Constraints
+
+- Keep backend code under `backend/src/Socialize.Api`.
+- Keep workspace feature code under `Modules/Workspaces`.
+- Do not expose raw token values in manager invite lists.
+- Frontend invite screens are covered by task 003 and task 004.
+
+## Likely Files
+
+- `backend/src/Socialize.Api/Modules/Workspaces/Data/WorkspaceInvite.cs`
+- `backend/src/Socialize.Api/Modules/Workspaces/Data/WorkspaceInviteStatuses.cs`
+- `backend/src/Socialize.Api/Modules/Workspaces/Data/WorkspaceModelConfiguration.cs`
+- `backend/src/Socialize.Api/Modules/Workspaces/Handlers/CreateWorkspaceInvite.cs`
+- `backend/src/Socialize.Api/Modules/Workspaces/Handlers/GetWorkspaceInvites.cs`
+- `backend/src/Socialize.Api/Modules/Workspaces/Handlers/*Accept*Invite*.cs`
+- `backend/tests/Socialize.Tests/`
+
+## Validation
+
+```bash
+dotnet build backend/Socialize.slnx
+dotnet test backend/Socialize.slnx
+```