feat(ai): pgvector embedding infrastructure (#19)

src/InboxIntel.Domain/Entities/Email.cs

@@ -54,6 +54,13 @@ public class Email : AuditableEntity
     /// </summary>
     public NpgsqlTypes.NpgsqlTsVector? SearchVector { get; set; }
 
+    /// <summary>
+    /// Semantic-search embedding (pgvector, 768-dim for nomic-embed-text). Populated by the
+    /// embedding backfill worker when AI is enabled; null otherwise (search falls back to
+    /// lexical). Nullable so the InMemory test provider and AI-off deployments work unchanged.
+    /// </summary>
+    public Pgvector.Vector? Embedding { get; set; }
+
     public ICollection<EmailLabel> EmailLabels { get; set; } = new List<EmailLabel>();
     public ICollection<Attachment> Attachments { get; set; } = new List<Attachment>();
 }

src/InboxIntel.Domain/InboxIntel.Domain.csproj

@@ -4,7 +4,9 @@
     <AssemblyName>InboxIntel.Domain</AssemblyName>
   </PropertyGroup>
   <ItemGroup>
-    <!-- NpgsqlTypes.NpgsqlTsVector is used on the Email entity for FTS mapping. -->
+    <!-- NpgsqlTypes.NpgsqlTsVector (FTS) and Pgvector.Vector (semantic search) are used as
+         column types on the Email entity — same pragmatic precedent for both. -->
     <PackageReference Include="Npgsql" Version="8.0.3" />
+    <PackageReference Include="Pgvector" Version="0.2.0" />
   </ItemGroup>
 </Project>