docs: define organization account model

docs/TASKS/organizations/001-organization-domain-foundation.md

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+# Task: Organization domain foundation
+
+## Feature
+
+`docs/FEATURES/organizations.md`
+
+## Goal
+
+Add the backend foundation for organizations as the SaaS account boundary and make workspaces belong to an organization.
+
+## Context
+
+Current docs and code treat `Workspace` as the top-level boundary. The product model now requires `Organization` above workspace for billing, subscriptions, connectors, limits, and workspace ownership.
+
+Existing local data does not need to be preserved.
+
+## Scope
+
+- Add an `Organizations` backend module or follow the existing ownership pattern if organization code belongs with `Workspaces`.
+- Add an organization persistence model with name, slug/display identity, timestamps, and basic audit fields matching local conventions.
+- Require every workspace to belong to exactly one organization.
+- Update workspace create/list/detail APIs to include organization ownership.
+- Add APIs for current user organization list and organization detail.
+- Add backend validation that users cannot access organizations they have no relationship with.
+- Seed or development bootstrap data should create at least one organization and owned workspace.
+- Update OpenAPI after backend contracts change.
+
+## Constraints
+
+- Keep backend code under `backend/src/Socialize.Api`.
+- Preserve FastEndpoints module structure.
+- Do not implement billing provider integration in this task.
+- Do not implement connector storage in this task.
+- Do not implement full organization membership override behavior in this task.
+- Existing development data may be wiped; do not spend scope on compatibility migration behavior.
+
+## Likely Files
+
+- `backend/src/Socialize.Api/Data/AppDbContext.cs`
+- `backend/src/Socialize.Api/Modules/Organizations/**`
+- `backend/src/Socialize.Api/Modules/Workspaces/**`
+- `backend/src/Socialize.Api/Migrations/**`
+- `backend/tests/Socialize.Tests/**`
+- `shared/openapi/openapi.json`
+- `frontend/src/api/schema.d.ts`
+
+## Done When
+
+- [ ] Organization entity is persisted.
+- [ ] Workspace requires `OrganizationId`.
+- [ ] Workspace APIs expose organization ownership.
+- [ ] Current user can list accessible organizations.
+- [ ] Current user can get accessible organization details.
+- [ ] Unauthorized organization access is rejected.
+- [ ] Development seed data creates an organization with owned workspaces.
+- [ ] Backend build and tests pass.
+- [ ] OpenAPI and generated frontend schema are updated.
+
+## Validation Commands
+
+```bash
+dotnet build backend/Socialize.slnx
+dotnet test backend/Socialize.slnx
+./scripts/update-openapi.sh
+```