Add database indexes for user-scoped queries

Tier 2 — scale

Problem

Every query in the app filters by UserId, and there is no index on UserId for LearningResources, ContentIdeas or SharedResourceGroups. The only indexes configured are the unique ones on ShareToken and invitation Token. Every page load is a table scan — and because thumbnails live on the same row (see #45), each scan drags the blobs with it.

Evidence

• LearnStack.Core/Data/ApplicationDbContext.cs — OnModelCreating defines indexes only on SharedResourceGroup.ShareToken, FriendInvitation.Token and the friendship pair

Proposed fix

Add in a migration:

• LearningResources (UserId, IsArchived, Status)
• LearningResources (UserId, IsPublic) filtered on IsPublic = 1
• ContentIdeas (UserId, Status)
• SharedResourceGroups (UserId)

Then confirm with actual execution plans on a seeded dataset.

Acceptance criteria

• No table scans on the resource list, Pulse or content-ideas pages
• Index usage verified against a dataset of ~10k resources

Start in LearnStack.Core/Data/ApplicationDbContext.cs by reviewing OnModelCreating and then inspect the existing migration setup. Apply the proposed indexes, seed roughly 10k resources, and compare execution plans for the resource list, Pulse, and content-ideas queries. Done means those queries show index usage with no table scans.