Fixes#76
## Why
Every proxied request spawned a goroutine running a 5s-timeout single-row INSERT. Under load this is unbounded goroutines and connection-pool pressure.
## Changes
- Add `database.AccessLogEntry` + `InsertAccessLogBatch` (bulk `COPY`).
- The engine starts one background writer that drains a buffered channel and flushes every 128 entries or 2s.
- `logAccess` is now a non-blocking channel send (drops on full buffer), so the request path never blocks on the DB. Best-effort telemetry: a small tail may be lost on abrupt shutdown.
## Validation
- `make e2e` passes.
Reviewed-on: #91
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>