Add optimistic concurrency (RowVersion) to user-editable entities

Tier 2 — correctness

Problem

No entity carries a concurrency token, and the update services copy every field from the incoming object onto the tracked entity. Two browser tabs — very common in a Blazor Server app, and the resource list is also edited from dialogs — silently overwrite each other, last write wins. Drag-and-drop reordering makes this worse because it rewrites CustomOrder across many rows at once.

Evidence

• LearnStack.Core/Data/Models/LearningResource.cs — no RowVersion
• LearnStack.Core/Services/LearningResourceService.cs:105-130 — wholesale field copy
• LearnStack.Core/Services/LearningResourceService.cs:157-176 — bulk CustomOrder rewrite

Proposed fix

1. Add [Timestamp] public byte[]? RowVersion { get; set; } to LearningResource and ContentIdea.
2. Handle DbUpdateConcurrencyException in the services and surface a "this item changed elsewhere" message instead of silently overwriting.

Acceptance criteria

• Concurrent edits produce a conflict message rather than silent data loss
• Reordering is resilient to a stale client-side list

Start with LearnStack.Core/Data/Models/LearningResource.cs and LearnStack.Core/Services/LearningResourceService.cs:105-130, then trace the service's bulk reorder path at lines 157-176. Identify the corresponding ContentIdea model and service, and verify how DbUpdateConcurrencyException can reach the user-facing flow. Done means concurrent edits show a conflict message and stale client-side reordering no longer silently loses changes.