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>

internal/database/migration_helpers_test.go

@@ -0,0 +1,82 @@
+package database
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// --- sortMigrationNames ---
+
+func TestSortMigrationNames_Alphabetical(t *testing.T) {
+	input := []string{"charlie", "alpha", "bravo"}
+	result := sortMigrationNames(input)
+
+	assert.Equal(t, []string{"alpha", "bravo", "charlie"}, result)
+	// Verify original slice is not mutated
+	assert.Equal(t, []string{"charlie", "alpha", "bravo"}, input)
+}
+
+func TestSortMigrationNames_Empty(t *testing.T) {
+	result := sortMigrationNames([]string{})
+	assert.Equal(t, []string{}, result)
+	assert.Len(t, result, 0)
+}
+
+// --- buildAppliedSet ---
+
+func TestBuildAppliedSet_Multiple(t *testing.T) {
+	applied := []DataMigration{
+		{ID: 1, Name: "20250101_first", AppliedAt: time.Now()},
+		{ID: 2, Name: "20250201_second", AppliedAt: time.Now()},
+		{ID: 3, Name: "20250301_third", AppliedAt: time.Now()},
+	}
+
+	set := buildAppliedSet(applied)
+
+	assert.Len(t, set, 3)
+	assert.True(t, set["20250101_first"])
+	assert.True(t, set["20250201_second"])
+	assert.True(t, set["20250301_third"])
+	assert.False(t, set["nonexistent"])
+}
+
+func TestBuildAppliedSet_Empty(t *testing.T) {
+	set := buildAppliedSet([]DataMigration{})
+	assert.Len(t, set, 0)
+}
+
+// --- filterPending ---
+
+func TestFilterPending_SomePending(t *testing.T) {
+	names := []string{"20250101_first", "20250201_second", "20250301_third"}
+	applied := map[string]bool{
+		"20250101_first": true,
+	}
+
+	pending := filterPending(names, applied)
+
+	assert.Equal(t, []string{"20250201_second", "20250301_third"}, pending)
+}
+
+func TestFilterPending_AllApplied(t *testing.T) {
+	names := []string{"20250101_first", "20250201_second"}
+	applied := map[string]bool{
+		"20250101_first":  true,
+		"20250201_second": true,
+	}
+
+	pending := filterPending(names, applied)
+
+	assert.Nil(t, pending)
+}
+
+func TestFilterPending_NoneApplied(t *testing.T) {
+	names := []string{"20250101_first", "20250201_second", "20250301_third"}
+	applied := map[string]bool{}
+
+	pending := filterPending(names, applied)
+
+	assert.Equal(t, []string{"20250101_first", "20250201_second", "20250301_third"}, pending)
+}