fix(import): handle Radio.create_contact :duplicate return in worker

apply_valid_rows only matched {:ok, _} and {:error, %Changeset{}}, so when
Radio.create_contact returned {:error, :duplicate, existing} — the race
window where preview flagged a row as new but another writer inserts the
same contact before the worker picks up the chunk — the reduce callback
crashed with FunctionClauseError. Oban retried the chunk, rolling back
the Postgrex transaction and surfacing the DBConnection errors the
operator was seeing.

Treat :duplicate as a silent skip: the row is already in the DB, there
is nothing to insert, and the counter should not double-count the existing
contact as an error either.

lib/microwaveprop/radio/csv_import.ex

@@ -326,6 +326,9 @@ defmodule Microwaveprop.Radio.CsvImport do
           ContactWeatherEnqueueWorker.enqueue_for_contact(contact)
           %{acc | imported: [contact | acc.imported]}
 
+        {:error, :duplicate, _existing} ->
+          acc
+
         {:error, %Ecto.Changeset{} = changeset} ->
           %{acc | errors: [{row.row_num, changeset_error_strings(changeset)} | acc.errors]}
       end

test/microwaveprop/workers/contact_import_worker_test.exs

@@ -219,4 +219,129 @@ defmodule Microwaveprop.Workers.ContactImportWorkerTest do
                })
     end
   end
+
+  # Reproduces a bug where the preview screen correctly flags an upload row
+  # as a duplicate of an existing DB contact (via direction-agnostic matching)
+  # but the async worker ends up inserting it anyway. The full pipeline —
+  # preview -> enqueue -> worker -> DB — must honor the preview's exclusion
+  # so the Imported count on /imports/:id never exceeds what the preview
+  # claimed as "Valid".
+  describe "preview-to-worker dedup parity" do
+    test "worker does not insert rows the preview classified as duplicates" do
+      {:ok, _existing} =
+        %Contact{}
+        |> Contact.submission_changeset(%{
+          "station1" => "W5XD",
+          "station2" => "K5TR",
+          "grid1" => "EM12",
+          "grid2" => "EM00",
+          "band" => "10000",
+          "mode" => "CW",
+          "qso_timestamp" => "2026-03-28T18:00:00Z",
+          "submitter_email" => @submitter
+        })
+        |> Repo.insert()
+
+      # Upload row is a direction-swapped version of the existing contact
+      # at the same time/band — should be flagged as a duplicate.
+      csv = build_csv(["K5TR,W5XD,EM00,EM12,10000,CW,2026-03-28T18:30:00Z"])
+
+      {:ok, preview} = CsvImport.preview(csv, @submitter)
+      assert preview.valid == []
+      assert length(preview.duplicates) == 1
+
+      {:ok, run_id} = CsvImport.enqueue(preview, @submitter)
+
+      run = Repo.get!(ImportRun, run_id)
+      assert run.total_rows == 0
+      assert run.rows["rows"] == []
+
+      assert Repo.aggregate(Contact, :count) == 1
+    end
+
+    test "worker does not insert in-upload direction-swapped duplicates" do
+      # Same contact logged from both ends — the CSV has row A->B and
+      # B->A at the same time/band. Only one should land in the DB.
+      csv =
+        build_csv([
+          "W5XD,K5TR,EM12,EM00,10000,CW,2026-03-28T18:00:00Z",
+          "K5TR,W5XD,EM00,EM12,10000,CW,2026-03-28T18:00:00Z"
+        ])
+
+      {:ok, preview} = CsvImport.preview(csv, @submitter)
+      assert length(preview.valid) == 1
+      assert length(preview.duplicates) == 1
+
+      {:ok, run_id} = CsvImport.enqueue(preview, @submitter)
+
+      run = Repo.get!(ImportRun, run_id)
+      assert run.total_rows == 1
+      assert length(run.rows["rows"]) == 1
+
+      assert Repo.aggregate(Contact, :count) == 1
+    end
+
+    # Covers the race window: preview ran when no matching contact existed,
+    # but by the time the worker picks up the chunk another insert (from a
+    # concurrent upload, a retry, etc.) has already landed. `Radio.create_contact`
+    # now returns `{:error, :duplicate, existing}`. The worker must not crash,
+    # must not double-insert, and must keep the counters in a sensible state.
+    test "worker survives when a row's duplicate appears in DB after preview" do
+      {:ok, _existing} =
+        %Contact{}
+        |> Contact.submission_changeset(%{
+          "station1" => "W5XD",
+          "station2" => "K5TR",
+          "grid1" => "EM12",
+          "grid2" => "EM00",
+          "band" => "10000",
+          "mode" => "CW",
+          "qso_timestamp" => "2026-03-28T18:00:00Z",
+          "submitter_email" => @submitter
+        })
+        |> Repo.insert()
+
+      # Hand-rolled ImportRun: simulates a preview where the row slipped
+      # through as "valid" (dup appeared after preview ran).
+      rows = [
+        %{
+          "kind" => "insert",
+          "row_num" => 2,
+          "attrs" => %{
+            "station1" => "W5XD",
+            "station2" => "K5TR",
+            "grid1" => "EM12",
+            "grid2" => "EM00",
+            "band" => "10000",
+            "mode" => "CW",
+            "qso_timestamp" => "2026-03-28T18:00:00Z",
+            "submitter_email" => @submitter
+          },
+          "timestamp" => "2026-03-28T18:00:00Z"
+        }
+      ]
+
+      {:ok, run} =
+        %ImportRun{}
+        |> ImportRun.changeset(%{
+          submitter_email: @submitter,
+          rows: %{"rows" => rows, "refinement_ids" => []},
+          total_rows: 1
+        })
+        |> Repo.insert()
+
+      assert :ok =
+               ContactImportWorker.perform(%Oban.Job{
+                 args: %{"import_run_id" => run.id, "offset" => 0, "limit" => 1}
+               })
+
+      reloaded = Repo.get!(ImportRun, run.id)
+      assert reloaded.processed_rows == 1
+      assert reloaded.imported_count == 0
+      assert reloaded.status == "complete"
+
+      # The existing contact must remain; no new row inserted.
+      assert Repo.aggregate(Contact, :count) == 1
+    end
+  end
 end