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/repository/logstore/dialect_postgres.go

@@ -0,0 +1,33 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package logstore
+
+import (
+	"strconv"
+)
+
+// timeBucketSQLPostgres 返回 PG 时间分桶表达式（epoch 秒 -> 分桶起点，int64）。
+func timeBucketSQLPostgres(column string, bucketSeconds int64) string {
+	return "(floor(extract(epoch from " + column + ")/" + strconv.FormatInt(bucketSeconds, 10) + ")*" + strconv.FormatInt(bucketSeconds, 10) + ")::bigint"
+}
+
+// dailyTrendDateSQLPostgres 返回 PG 按日聚合的日期表达式。
+func dailyTrendDateSQLPostgres() string {
+	return "to_char(created_at, 'YYYY-MM-DD')"
+}
+
+// epochSQLPostgres 返回 PG epoch 秒表达式（int64）。
+func epochSQLPostgres(column string) string {
+	return "extract(epoch from " + column + ")::bigint"
+}
+
+// textCastSQLPostgres 返回 PG 数值列转文本表达式。
+func textCastSQLPostgres(column string) string {
+	return column + "::text"
+}
+
+// distinctNonEmptyCountSQLPostgres 返回 PG 排除空串的 distinct 计数表达式。
+func distinctNonEmptyCountSQLPostgres(column string) string {
+	return "COUNT(DISTINCT " + column + ") FILTER (WHERE " + column + " <> '')"
+}