feat(db): add ClickHouse batchwriter framework and skill

Introduce internal/db/batchwriter as a reusable generic buffered writer
for per-domain ClickHouse flush pipelines, with unit tests and default
batch tuning aligned with audit log ingestion.

Add clickhouse-batchwriter agent skill and cross-references in AGENTS.md
and database-migration. Business layers are not wired yet.

internal/db/batchwriter/config.go

@@ -0,0 +1,52 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package batchwriter
+
+import (
+	"fmt"
+	"time"
+)
+
+const (
+	defaultQueueSize    = 10_000
+	defaultMaxBatchSize = 1_000
+	defaultFlushEvery   = time.Second
+)
+
+// Config controls queue capacity and flush thresholds for a Writer instance.
+type Config struct {
+	// Name identifies the writer in logs and diagnostics. Optional.
+	Name string
+
+	// QueueSize is the buffered channel capacity.
+	QueueSize int
+
+	// MaxBatchSize triggers a flush when the in-memory batch reaches this count.
+	MaxBatchSize int
+
+	// FlushInterval triggers a time-based flush even when the batch is smaller.
+	FlushInterval time.Duration
+}
+
+// DefaultConfig returns production-friendly defaults aligned with audit log batching.
+func DefaultConfig() Config {
+	return Config{
+		QueueSize:     defaultQueueSize,
+		MaxBatchSize:  defaultMaxBatchSize,
+		FlushInterval: defaultFlushEvery,
+	}
+}
+
+func (c Config) validate() error {
+	if c.QueueSize <= 0 {
+		return fmt.Errorf("batchwriter: queue size must be positive")
+	}
+	if c.MaxBatchSize <= 0 {
+		return fmt.Errorf("batchwriter: max batch size must be positive")
+	}
+	if c.FlushInterval <= 0 {
+		return fmt.Errorf("batchwriter: flush interval must be positive")
+	}
+	return nil
+}