Coverage priorities 1-5: test pure functions, extract interfaces, mock-based handler tests

- Priority 1: Test NewSendEmailTask + NewSendPushTask (5 tests)
- Priority 2: Test customHTTPErrorHandler — all 15+ branches (21 tests)
- Priority 3: Extract Enqueuer interface + payload builders in worker pkg (5 tests)
- Priority 4: Extract ClassifyFile/ComputeRelPath in migrate-encrypt (6 tests)
- Priority 5: Define Handler interfaces, refactor to accept them, mock-based tests (14 tests)
- Fix .gitignore: /worker instead of worker to stop ignoring internal/worker/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/backfill-completion-columns/main_test.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+func TestClassifyCompletion_CompletedAfterDue_ReturnsOverdue(t *testing.T) {
+	due := time.Date(2025, 6, 1, 0, 0, 0, 0, time.UTC)
+	completed := time.Date(2025, 6, 5, 14, 0, 0, 0, time.UTC)
+	got := classifyCompletion(completed, due, 7)
+	if got != "overdue_tasks" {
+		t.Errorf("got %q, want overdue_tasks", got)
+	}
+}
+
+func TestClassifyCompletion_CompletedOnDueDate_ReturnsDueSoon(t *testing.T) {
+	due := time.Date(2025, 6, 1, 0, 0, 0, 0, time.UTC)
+	completed := time.Date(2025, 6, 1, 10, 0, 0, 0, time.UTC)
+	got := classifyCompletion(completed, due, 7)
+	if got != "due_soon_tasks" {
+		t.Errorf("got %q, want due_soon_tasks", got)
+	}
+}
+
+func TestClassifyCompletion_CompletedWithinThreshold_ReturnsDueSoon(t *testing.T) {
+	due := time.Date(2025, 6, 10, 0, 0, 0, 0, time.UTC)
+	completed := time.Date(2025, 6, 5, 0, 0, 0, 0, time.UTC) // 5 days before due, threshold 7
+	got := classifyCompletion(completed, due, 7)
+	if got != "due_soon_tasks" {
+		t.Errorf("got %q, want due_soon_tasks", got)
+	}
+}
+
+func TestClassifyCompletion_CompletedAtExactThreshold_ReturnsDueSoon(t *testing.T) {
+	due := time.Date(2025, 6, 10, 0, 0, 0, 0, time.UTC)
+	completed := time.Date(2025, 6, 3, 0, 0, 0, 0, time.UTC) // exactly 7 days before due
+	got := classifyCompletion(completed, due, 7)
+	if got != "due_soon_tasks" {
+		t.Errorf("got %q, want due_soon_tasks", got)
+	}
+}
+
+func TestClassifyCompletion_CompletedBeyondThreshold_ReturnsUpcoming(t *testing.T) {
+	due := time.Date(2025, 6, 30, 0, 0, 0, 0, time.UTC)
+	completed := time.Date(2025, 6, 1, 0, 0, 0, 0, time.UTC) // 29 days before due, threshold 7
+	got := classifyCompletion(completed, due, 7)
+	if got != "upcoming_tasks" {
+		t.Errorf("got %q, want upcoming_tasks", got)
+	}
+}
+
+func TestClassifyCompletion_TimeNormalization_SameDayDifferentTimes(t *testing.T) {
+	due := time.Date(2025, 6, 1, 23, 59, 59, 0, time.UTC)
+	completed := time.Date(2025, 6, 1, 0, 0, 1, 0, time.UTC) // same day, different times
+	got := classifyCompletion(completed, due, 7)
+	// Same day → daysBefore == 0 → within threshold → due_soon
+	if got != "due_soon_tasks" {
+		t.Errorf("got %q, want due_soon_tasks", got)
+	}
+}