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_sqlite.go

@@ -0,0 +1,85 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package logstore
+
+import (
+	"strconv"
+
+	"gorm.io/gorm"
+)
+
+// isPostgresDialect 判断 gorm 句柄是否为 PostgreSQL 方言（否则按 SQLite 处理）。
+// Dialector 经 gorm.Config 内嵌提升，Name() 可直接在 DB 上调用。
+func isPostgresDialect(db *gorm.DB) bool {
+	return db != nil && db.Dialector != nil && db.Name() == "postgres"
+}
+
+// timeBucketSQLSQLite 返回 SQLite 时间分桶表达式（epoch 秒 -> 分桶起点）。
+func timeBucketSQLSQLite(column string, bucketSeconds int64) string {
+	return "(floor(unixepoch(" + column + ")/" + strconv.FormatInt(bucketSeconds, 10) + ")*" + strconv.FormatInt(bucketSeconds, 10) + ")"
+}
+
+// dailyTrendDateSQLSQLite 返回 SQLite 按日聚合的日期表达式。
+func dailyTrendDateSQLSQLite() string {
+	return "strftime('%Y-%m-%d', created_at)"
+}
+
+// epochSQLSQLite 返回 SQLite epoch 秒表达式（unixepoch 整数秒）。
+func epochSQLSQLite(column string) string {
+	return "unixepoch(" + column + ")"
+}
+
+// textCastSQLSQLite 返回 SQLite 数值列转文本表达式。
+func textCastSQLSQLite(column string) string {
+	return "CAST(" + column + " AS TEXT)"
+}
+
+// distinctNonEmptyCountSQLSQLite 返回 SQLite 排除空串的 distinct 计数表达式
+// （SQLite 无 FILTER 语法，用 CASE 等价实现）。
+func distinctNonEmptyCountSQLSQLite(column string) string {
+	return "COUNT(DISTINCT CASE WHEN " + column + " <> '' THEN " + column + " END)"
+}
+
+// distinctNonEmptyCountSQL 按当前方言返回排除空串的 distinct 计数表达式
+// （运行时按 Dialector 分发，默认 SQLite）。
+func distinctNonEmptyCountSQL(db *gorm.DB, column string) string {
+	if isPostgresDialect(db) {
+		return distinctNonEmptyCountSQLPostgres(column)
+	}
+	return distinctNonEmptyCountSQLSQLite(column)
+}
+
+// dailyTrendDateSQL 按当前方言返回按日聚合的日期表达式（运行时按 Dialector 分发，默认 SQLite）。
+func dailyTrendDateSQL(db *gorm.DB) string {
+	if isPostgresDialect(db) {
+		return dailyTrendDateSQLPostgres()
+	}
+	return dailyTrendDateSQLSQLite()
+}
+
+// epochSQL 按当前方言返回 epoch 秒表达式（运行时按 Dialector 分发，默认 SQLite）。
+func epochSQL(db *gorm.DB, column string) string {
+	if isPostgresDialect(db) {
+		return epochSQLPostgres(column)
+	}
+	return epochSQLSQLite(column)
+}
+
+// textCastSQL 按当前方言返回数值列转文本表达式（运行时按 Dialector 分发，默认 SQLite）。
+func textCastSQL(db *gorm.DB, column string) string {
+	if isPostgresDialect(db) {
+		return textCastSQLPostgres(column)
+	}
+	return textCastSQLSQLite(column)
+}
+
+// timeBucketSQL 按当前方言返回时间分桶表达式。
+// brief 将 PG/SQLite 两版写为同名函数，同包无法共存；log_database 为运行时配置，
+// 不能使用编译期 build tag，故按 db.Dialector.Name() 运行时分发（默认 SQLite）。
+func timeBucketSQL(db *gorm.DB, column string, bucketSeconds int64) string {
+	if isPostgresDialect(db) {
+		return timeBucketSQLPostgres(column, bucketSeconds)
+	}
+	return timeBucketSQLSQLite(column, bucketSeconds)
+}