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.go

@@ -0,0 +1,31 @@
+package database
+
+import "sort"
+
+// sortMigrationNames returns a sorted copy of the names slice.
+func sortMigrationNames(names []string) []string {
+	sorted := make([]string, len(names))
+	copy(sorted, names)
+	sort.Strings(sorted)
+	return sorted
+}
+
+// buildAppliedSet converts a list of applied migrations to a lookup set.
+func buildAppliedSet(applied []DataMigration) map[string]bool {
+	set := make(map[string]bool, len(applied))
+	for _, m := range applied {
+		set[m.Name] = true
+	}
+	return set
+}
+
+// filterPending returns names not present in the applied set.
+func filterPending(names []string, applied map[string]bool) []string {
+	var pending []string
+	for _, name := range names {
+		if !applied[name] {
+			pending = append(pending, name)
+		}
+	}
+	return pending
+}