feat(backend): add semantic matchmaking with embedding pipeline

Add a pluggable embedding provider with an offline deterministic default, vector indexing for problems and innovations, cosine semantic ranking, similar cases and solution suggestions, area filtering and an admin reindex endpoint. Refresh the index when innovations or submissions change.

backend/Infrastructure/DependencyInjection.cs

@@ -2,6 +2,7 @@ using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using SurrealDb.Net;
 using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Matchmaking;
 using TarninaHub.Infrastructure.Auth;
 using TarninaHub.Infrastructure.Events;
 using TarninaHub.Infrastructure.Persistence;
@@ -59,6 +60,8 @@ public static class DependencyInjection
         services.AddSingleton<IKnowledgeItemService, KnowledgeItemService>();
         services.AddSingleton<IInnovationService, InnovationService>();
         services.AddSingleton<ITrendService, TrendService>();
+        services.AddSingleton<IEmbeddingProvider, HashingEmbeddingProvider>();
+        services.AddSingleton<IMatchmakingService, MatchmakingService>();
 
         return services;
     }