perf: batch access-log writes instead of goroutine+insert per request (#91)

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>

internal/database/artifacts.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"time"
 
+	"github.com/jackc/pgx/v5"
+
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
@@ -109,6 +111,33 @@ func (db *DB) InsertAccessLog(ctx context.Context, remoteName, path string, cach
 	return err
 }
 
+// AccessLogEntry is one buffered access-log record.
+type AccessLogEntry struct {
+	RemoteName string
+	Path       string
+	CacheHit   bool
+	SizeBytes  int64
+	UpstreamMS int
+	ClientIP   string
+}
+
+// InsertAccessLogBatch bulk-inserts access-log rows with a single COPY.
+func (db *DB) InsertAccessLogBatch(ctx context.Context, entries []AccessLogEntry) error {
+	if len(entries) == 0 {
+		return nil
+	}
+	rows := make([][]any, len(entries))
+	for i, e := range entries {
+		rows[i] = []any{e.RemoteName, e.Path, e.CacheHit, e.SizeBytes, e.UpstreamMS, e.ClientIP}
+	}
+	_, err := db.Pool.CopyFrom(ctx,
+		pgx.Identifier{"access_log"},
+		[]string{"remote_name", "path", "cache_hit", "size_bytes", "upstream_ms", "client_ip"},
+		pgx.CopyFromRows(rows),
+	)
+	return err
+}
+
 func (db *DB) FindOrphanedBlobs(ctx context.Context) ([]models.Blob, error) {
 	rows, err := db.Pool.Query(ctx, `
 		SELECT b.content_hash, b.s3_key, b.size_bytes, b.content_type, b.created_at