Vikunja: Assignee email addresses disclosed to read-only project members via the task assignees endpoint

Summary

### Summary `TaskAssginee.ReadAll` returns assignee user objects without blanking the `Email` field, disclosing assignee email addresses to any read-only project member. Every sibling path that returns user objects obfuscates the email; this one does not.

### Details `pkg/models/task_assignees.go` (~lines 306-344) does `Select("users.*")` and returns the result directly. `User.Email` is `json:"email,omitempty"`, so a non-empty value always serializes. The endpoint gates on `task.CanRead`, so a read-only member passes. Sibling paths blank the field: `pkg/models/tasks.go:530`, `pkg/models/project_users.go:216`, `pkg/models/teams.go:177`, `pkg/models/label_task.go:312`, `pkg/models/task_attachment.go:511`. The omission here reads as an oversight, not a decision.

The same file's `getRawTaskAssigneesForTasks` (~line 56) also selects `users.*` but is safe because its only caller (`addAssigneesToTasks`) blanks the email afterwards.

### PoC (verified at runtime against v2.5.0, v1 and v2) ``` GET /api/v1/tasks/{id}/assignees (reader with permission:0) -> [{"id":37,"username":"...","email":"assignee+SECRET@example.test", ...}] ``` Same leak on `GET /api/v2/tasks/{id}/assignees` (routes through the identical model method). Contrast: `GET /api/v1/projects/{id}/projectusers` and the project task-embed both return the same users with no email.

### Impact Disclosure of assignees' email addresses to users who should only see usernames. Read-only.

### Fix Blank `Email` on each returned user in `TaskAssginee.ReadAll` before returning, matching the sibling paths. Covers v1 and v2 at once.