fix: replace string(rune()) with strconv.Itoa in tests (bd-fmc)

Also updated CONFIG.md to clarify mass delete threshold requires >5 issues (bd-in6).

The string(rune('0'+i)) pattern produces incorrect characters when i >= 10.
Changed to strconv.Itoa(i) for reliable conversion.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/storage/sqlite/compact_bench_test.go

@@ -4,6 +4,7 @@ package sqlite
 
 import (
 	"context"
+	"strconv"
 	"testing"
 	"time"
 
@@ -112,9 +113,9 @@ func BenchmarkCheckEligibility(b *testing.B) {
 	}
 }
 
-func generateID(b testing.TB, prefix string, n int) string{
+func generateID(b testing.TB, prefix string, n int) string {
 	b.Helper()
-	return prefix + string(rune('0'+n/10)) + string(rune('0'+n%10))
+	return prefix + strconv.Itoa(n)
 }
 
 func setupBenchDB(tb testing.TB) (*SQLiteStorage, func()) {