refactor(migration): delete pkg/migrator, move SQL to per-plugin embed

BREAKING: pkg/migrator/ deleted entirely. Migration SQL files are now
owned by each plugin in its own migrations/ directory.

Architecture:
- Delete pkg/migrator/ (26 global SQL files + ClickHouse migration)
- Move global SQL to per-plugin migrations/ with go:embed + Register()
- Rewrite cmd/app.go gooseEngine: uses Inject[DBService] for DB, iterates
  all plugin-registered MigrationEntry, runs goose.Up per entry
- core.MigrationEngine.Migrate signature changed: *Context instead of
  context.Context, so engine can resolve services via IoC

Per-plugin migration ownership:
  auth/             → w_access_tokens, w_auth_sources, w_external_accounts
  user/             → w_users (seed system user)
  admin/            → w_system_configs, w_templates (seeds)
  upload/           → w_uploads, w_upload_stats
  message_gateway/  → w_push_*, w_message_*
  risk_control/     → w_user_access_logs (PG + ClickHouse)
  driver_asynq_cron/  → w_schedules
  driver_asynq_worker/ → w_task_executions

Dependencies:
- cmd/banner.go: removed migration report display (migrations are automatic)
- cmd/reset_passwd.go: removed PreRun migrator.Migrate() call
- go.mod: clickhouse-go kept (used by plugins/infra/database/clickhouse.go)

plugins/domain/auth/migrations/00002_add_access_token_is_admin.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+ALTER TABLE w_access_tokens ADD COLUMN IF NOT EXISTS is_admin BOOLEAN NOT NULL DEFAULT FALSE;
+
+-- +goose Down
+ALTER TABLE w_access_tokens DROP COLUMN IF EXISTS is_admin;

plugins/domain/auth/migrations/00003_drop_access_token_last_used_at.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+ALTER TABLE w_access_tokens DROP COLUMN IF EXISTS last_used_at;
+
+-- +goose Down
+ALTER TABLE w_access_tokens ADD COLUMN last_used_at TIMESTAMPTZ;

plugins/domain/auth/migrations/00004_seed_login_session_ttl.sql

@@ -0,0 +1,8 @@
+-- +goose Up
+INSERT INTO w_system_configs (key, value, type, visibility, description, created_at, updated_at)
+VALUES
+  ('login_session_ttl_hours', '0', 'system', 0, '登录会话过期时间 (小时，0表示浏览器关闭后自动退出，-1表示永不过期)', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+ON CONFLICT (key) DO NOTHING;
+
+-- +goose Down
+DELETE FROM w_system_configs WHERE key = 'login_session_ttl_hours';