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/model/analytics/node_observability.go

@@ -90,6 +90,34 @@ type AccessLogHourly struct {
 	RequestLength int64     `gorm:"column:request_length"`
 }
 
+// NodeTrafficHourly is an hourly traffic rollup row.
+//
+// UniqueVisitorCount is always 0 when sourced from of_access_log_hourly
+// (true UV requires raw uniqExact on access logs).
+type NodeTrafficHourly struct {
+	NodeID             string
+	Hour               time.Time
+	RequestCount       int64
+	ErrorCount         int64
+	UniqueVisitorCount int64
+}
+
+// NodeMetricHourly is an hourly metric snapshot aggregation row.
+//
+// Disk and host network counters are cumulative. Prefer pre-aggregated min/max
+// deltas from of_node_metric_capacity_hourly; raw fallback uses consecutive
+// lagInFrame samples per node (negative deltas after counter reset are dropped).
+type NodeMetricHourly struct {
+	Hour                      time.Time
+	AverageCPUUsagePercent    float64
+	AverageMemoryUsagePercent float64
+	NetworkRxBytes            int64
+	NetworkTxBytes            int64
+	DiskReadBytes             int64
+	DiskWriteBytes            int64
+	ReportedNodes             int
+}
+
 // NodeObsFrps stores FRPS observability snapshots in ClickHouse.
 type NodeObsFrps struct {
 	ID              uint64    `gorm:"column:id"`