Fix HF dataset import internal error and training SQLite readiness.

Wrap AssistentImportHfDataset in try/catch, batch inserts in a transaction, clearer HF rows errors for gated datasets, EnsureTrainingReady with actionable message, and friendlier training UI errors instead of generic internal error.

Co-authored-by: Cursor <cursoragent@cursor.com>

AssistentMemory.cs

@@ -149,7 +149,7 @@ public sealed partial class AssistentMemory : IDisposable
         }
         catch (Exception ex)
         {
-            Logs.Debug($"AssistentMemory training schema: {ex.Message}");
+            Logs.Error($"AssistentMemory training schema failed: {ex.Message}");
         }
         try
         {
@@ -166,6 +166,10 @@ public sealed partial class AssistentMemory : IDisposable
 
     bool HasColumn(string table, string column)
     {
+        if (!HasTable(table))
+        {
+            return false;
+        }
         using SqliteCommand cmd = _conn.CreateCommand();
         cmd.CommandText = $"PRAGMA table_info({table})";
         using SqliteDataReader reader = cmd.ExecuteReader();
@@ -179,6 +183,29 @@ public sealed partial class AssistentMemory : IDisposable
         return false;
     }
 
+    bool HasTable(string table)
+    {
+        using SqliteCommand cmd = _conn.CreateCommand();
+        cmd.CommandText = "SELECT 1 FROM sqlite_master WHERE type = 'table' AND name = $n LIMIT 1";
+        cmd.Parameters.AddWithValue("$n", table ?? "");
+        return cmd.ExecuteScalar() != null;
+    }
+
+    internal void EnsureTrainingReady()
+    {
+        EnsureOpen();
+        if (!HasTable("train_samples"))
+        {
+            EnsureTrainingSchema();
+        }
+        if (!HasTable("train_samples"))
+        {
+            throw new InvalidOperationException(
+                "Assistent training database unavailable (train_samples). "
+                + "Run gpu-rent seed-extensions and restart SwarmUI (≥0.15.6).");
+        }
+    }
+
     void MigratePersonaColumn()
     {
         if (HasColumn("memories", "persona"))