feat(log): decouple log storage from ClickHouse with switchable logstore

- New internal/repository/logstore abstraction: exported domain interfaces
  (AccessLogStore/ObservabilityStore/UserAccessLogStore/StatusStore),
  config-driven provider (Active/Build/Migrating/SetConfigReader), GORM
  implementation for PostgreSQL/SQLite (incl. hourly rollups computed in
  real time, migration listers, PG partition maintenance), and a ClickHouse
  wrapper preserving the native batch path; repository facade delegates to
  logstore; import-lint test enforces apps never import analyticsrepo.
- ClickHouse is now optional: the log DB is either the main DB (postgres
  when database.enabled, else sqlite) or clickhouse; boot validation +
  first-run seed; log_database / log_db_migration are protected keys.
- New user task 切换日志数据库 (of_log_db_switch): freeze log writes,
  drain batch writers, copy all 6 raw log tables by id (preserving IDs)
  with target-partition pre-creation for PG, flip log_database on success,
  clear the freeze flag on failure.
- Per-store retention (log_retention_days_*) with expiry cleanup folded
  into the daily system_cleanup task; legacy database_auto_cleanup_* and
  of_database_auto_cleanup decommissioned.
- goose migrations: 6 log tables in PG (2 monthly-partitioned) + SQLite,
  retention config seeds, schedule cleanup; GET
  /api/v1/admin/status/log-database endpoint; frontend retention settings,
  switch-task UI and status badge; changelog and docs updated.

docs(plan): log database decoupling implementation plan

docs(design): log database decoupling design (ClickHouse optional)

internal/infra/persistence/batchwriter/writer.go

@@ -11,6 +11,8 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
+
+	"github.com/Rain-kl/Wavelet/internal/model/analytics"
 )
 
 // FlushFunc persists a batch of queued items. It is invoked from the worker goroutine.
@@ -22,14 +24,8 @@ type FlushFunc[T any] func(ctx context.Context, items []T) error
 type FlushErrorHandler[T any] func(ctx context.Context, items []T, err error)
 
 // Stats is a point-in-time snapshot of Writer queue and failure counters.
-type Stats struct {
-	Name        string `json:"name"`
-	Depth       int    `json:"depth"`
-	Cap         int    `json:"cap"`
-	Drops       int64  `json:"drops"`
-	FlushErrors int64  `json:"flush_errors"`
-	Running     bool   `json:"running"`
-}
+// It is an alias of analyticsmodel.BatchWriterStats (moved to keep model pure data).
+type Stats = analytics.BatchWriterStats
 
 // Writer buffers items and flushes them by size or interval.
 type Writer[T any] struct {