Decouple repository interfaces from transport DTOs and make database operations asynchronous #8

Problem

Repository interfaces depend on HTTP/Kafka contracts from LinkTracker.Shared:

• ILinkRepository
• ITagRepository
• Shared models

Repositories currently return LinkResponse and TagResponse, which are transport DTOs.

Repository operations are also synchronous, causing ASP.NET worker threads to block while waiting for PostgreSQL.

Proposed solution

• Introduce domain models or application projections for links, tags, and scheduled links.
• Move mapping to HTTP responses into the API layer.
• Convert repository methods to async methods.
• Add CancellationToken to all database operations.
• Preserve both SQL and EF Core implementations.
• Handle concurrent inserts safely in the ORM implementation.

Acceptance criteria

• Repository interfaces do not reference LinkTracker.Shared.Models.
• Repository implementations return domain/application models.
• All database operations use async Npgsql or EF Core APIs.
• All operations accept a CancellationToken.
• Concurrent link and tag creation does not cause an unhandled unique-constraint exception.
• The same repository contract tests pass for SQL and ORM implementations.