feat(ai): IEmbeddingProvider foundation (#18)

src/InboxIntel.Application/Abstractions/IServices.cs

@@ -82,6 +82,25 @@ public interface IAiProvider
     Task<string> CompleteAsync(string systemPrompt, string userPrompt, CancellationToken ct = default);
 }
 
+/// <summary>
+/// Produces vector embeddings for text (the foundation for semantic search, near-duplicate
+/// detection, and "find similar"). Kept separate from <see cref="IAiProvider"/> because
+/// embeddings are a distinct capability with their own model. The Null implementation returns
+/// an empty vector and <see cref="IsAvailable"/> = false, so callers detect unavailability and
+/// fall back to lexical search — AI is never required for core functionality.
+/// </summary>
+public interface IEmbeddingProvider
+{
+    /// <summary>False for the Null provider (AI off / no embedding model configured).</summary>
+    bool IsAvailable { get; }
+
+    /// <summary>Embed a single text. Returns an empty array when unavailable.</summary>
+    Task<float[]> EmbedAsync(string text, CancellationToken ct = default);
+
+    /// <summary>Embed many texts, result aligned to input order. Empty list when unavailable.</summary>
+    Task<IReadOnlyList<float[]>> EmbedBatchAsync(IReadOnlyList<string> texts, CancellationToken ct = default);
+}
+
 public enum ExportFormat { Pdf, Csv, Json }
 
 public interface IExportService