Fix statement leaks, error handling, UTC timestamps in Engram

lib/symbiont/engram.ex

@@ -503,27 +503,41 @@ defmodule Symbiont.Engram do
   end
 
   defp generate_session_id do
-    now = NaiveDateTime.local_now()
+    now = DateTime.utc_now()
     hex = :crypto.strong_rand_bytes(4) |> Base.encode16(case: :lower)
 
     now
-    |> NaiveDateTime.to_iso8601()
+    |> DateTime.to_iso8601()
     |> String.replace(~r/[T:\-]/, "")
     |> String.slice(0, 14)
     |> Kernel.<>("-#{hex}")
   end
 
   defp now_iso do
-    NaiveDateTime.local_now() |> NaiveDateTime.to_iso8601()
+    DateTime.utc_now() |> DateTime.to_iso8601()
   end
 
   defp is_stale?(last_heartbeat) do
-    case NaiveDateTime.from_iso8601(last_heartbeat) do
-      {:ok, dt} ->
-        NaiveDateTime.diff(NaiveDateTime.local_now(), dt, :minute) > @stale_threshold_minutes
+    # Parse timestamp — handles both UTC DateTime (new) and naive (legacy Python)
+    with {:error, _} <- parse_as_datetime(last_heartbeat),
+         {:error, _} <- parse_as_naive(last_heartbeat) do
+      false
+    else
+      {:ok, dt} -> DateTime.diff(DateTime.utc_now(), dt, :minute) > @stale_threshold_minutes
+    end
+  end
 
-      _ ->
-        false
+  defp parse_as_datetime(str) do
+    case DateTime.from_iso8601(str) do
+      {:ok, dt, _offset} -> {:ok, dt}
+      _ -> {:error, :invalid}
+    end
+  end
+
+  defp parse_as_naive(str) do
+    case NaiveDateTime.from_iso8601(str) do
+      {:ok, ndt} -> {:ok, DateTime.from_naive!(ndt, "Etc/UTC")}
+      _ -> {:error, :invalid}
     end
   end
 
@@ -546,49 +560,59 @@ defmodule Symbiont.Engram do
   defp exec(conn, sql, params \\ []) do
     {:ok, stmt} = Exqlite.Sqlite3.prepare(conn, sql)
 
-    if params != [] do
-      :ok = Exqlite.Sqlite3.bind(stmt, params)
-    end
+    try do
+      if params != [] do
+        :ok = Exqlite.Sqlite3.bind(stmt, params)
+      end
 
-    case Exqlite.Sqlite3.step(conn, stmt) do
-      :done -> :ok
-      {:row, _} -> :ok
-      {:error, reason} -> {:error, reason}
+      case Exqlite.Sqlite3.step(conn, stmt) do
+        :done -> :ok
+        {:row, _} -> :ok
+        {:error, reason} -> raise "SQLite exec error: #{inspect(reason)}"
+      end
+    after
+      Exqlite.Sqlite3.release(conn, stmt)
     end
-  after
-    :ok
   end
 
   defp query_one(conn, sql, params \\ []) do
     {:ok, stmt} = Exqlite.Sqlite3.prepare(conn, sql)
 
-    if params != [] do
-      :ok = Exqlite.Sqlite3.bind(stmt, params)
-    end
+    try do
+      if params != [] do
+        :ok = Exqlite.Sqlite3.bind(stmt, params)
+      end
 
-    case Exqlite.Sqlite3.step(conn, stmt) do
-      {:row, row} -> {:ok, row}
-      :done -> :empty
-      {:error, reason} -> {:error, reason}
+      case Exqlite.Sqlite3.step(conn, stmt) do
+        {:row, row} -> {:ok, row}
+        :done -> :empty
+        {:error, reason} -> raise "SQLite query error: #{inspect(reason)}"
+      end
+    after
+      Exqlite.Sqlite3.release(conn, stmt)
     end
   end
 
   defp query_all(conn, sql, params \\ []) do
     {:ok, stmt} = Exqlite.Sqlite3.prepare(conn, sql)
 
-    if params != [] do
-      :ok = Exqlite.Sqlite3.bind(stmt, params)
-    end
+    try do
+      if params != [] do
+        :ok = Exqlite.Sqlite3.bind(stmt, params)
+      end
 
-    rows = collect_rows(conn, stmt, [])
-    {:ok, rows}
+      rows = collect_rows(conn, stmt, [])
+      {:ok, rows}
+    after
+      Exqlite.Sqlite3.release(conn, stmt)
+    end
   end
 
   defp collect_rows(conn, stmt, acc) do
     case Exqlite.Sqlite3.step(conn, stmt) do
       {:row, row} -> collect_rows(conn, stmt, [row | acc])
       :done -> Enum.reverse(acc)
-      {:error, _reason} -> Enum.reverse(acc)
+      {:error, reason} -> raise "SQLite step error mid-collection: #{inspect(reason)}"
     end
   end
 end