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/upload/migrations/00001_add_access_mode.sql

@@ -0,0 +1,6 @@
+-- +goose Up
+ALTER TABLE w_uploads ADD COLUMN IF NOT EXISTS access_mode INTEGER NOT NULL DEFAULT 0;
+UPDATE w_uploads SET access_mode = 1 WHERE type = 'avatar';
+
+-- +goose Down
+ALTER TABLE w_uploads DROP COLUMN IF EXISTS access_mode;

plugins/domain/upload/migrations/00002_add_composite_indexes.sql

@@ -0,0 +1,7 @@
+-- +goose Up
+CREATE INDEX IF NOT EXISTS idx_w_uploads_status_created_at ON w_uploads (status, created_at);
+CREATE INDEX IF NOT EXISTS idx_w_uploads_hash_file_size_status ON w_uploads (hash, file_size, status);
+
+-- +goose Down
+DROP INDEX IF EXISTS idx_w_uploads_hash_file_size_status;
+DROP INDEX IF EXISTS idx_w_uploads_status_created_at;

plugins/domain/upload/migrations/00003_create_upload_stats.sql

@@ -0,0 +1,12 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS w_upload_stats (
+    dimension VARCHAR(32) NOT NULL,
+    stat_key VARCHAR(64) NOT NULL DEFAULT '',
+    file_count BIGINT NOT NULL DEFAULT 0,
+    file_size BIGINT NOT NULL DEFAULT 0,
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (dimension, stat_key)
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS w_upload_stats;

plugins/domain/upload/migrations/00004_backfill_upload_stats.sql

@@ -0,0 +1,12 @@
+-- +goose Up
+INSERT INTO w_upload_stats (dimension, stat_key, file_count, file_size)
+SELECT 'total', '', COUNT(*), COALESCE(SUM(file_size), 0)
+FROM w_uploads
+WHERE status != 'deleted'
+ON CONFLICT (dimension, stat_key) DO UPDATE SET
+    file_count = EXCLUDED.file_count,
+    file_size = EXCLUDED.file_size,
+    updated_at = CURRENT_TIMESTAMP;
+
+-- +goose Down
+DELETE FROM w_upload_stats;

plugins/domain/upload/migrations/00005_drop_storage_driver.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+ALTER TABLE w_uploads DROP COLUMN IF EXISTS storage_driver;
+
+-- +goose Down
+ALTER TABLE w_uploads ADD COLUMN IF NOT EXISTS storage_driver VARCHAR(50) NOT NULL DEFAULT 'local';