perf(accesslog): use buffer pool in BackScanner to reduce allocations

Replace per-scan byte slice allocations with a sized buffer pool,
significantly reducing memory pressure during log file scanning.

- Add Release() method to return buffers to pool (callers must invoke)
- Remove Reset() method - create new scanner instead for simpler lifecycle
- Refactor chunk prepending to reuse pooled buffers instead of append

Benchmark results show allocations dropped from ~26k to 1 per scan
for small chunk sizes, with better throughput.

BREAKING CHANGE: Reset() removed; callers must call Release() and
create a new BackScanner instance instead.

internal/logging/accesslog/retention.go

@@ -20,7 +20,7 @@ var (
 )
 
 // see back_scanner_test.go#L210 for benchmarks
-var defaultChunkSize = 256 * kilobyte
+var defaultChunkSize = 32 * kilobyte
 
 // Syntax:
 //