feat(migration): replace sqlx::migrate with standalone Go migrate tool

Schema is now managed by a separate Go CLI at tools/migrate/, which embeds
the SQL files copied from server/initialize/migrate/database/ and tracks
state in the shared schema_migrations table. Rust no longer owns migrations
at startup.

Rust startup:
  1. Connect to DB
  2. ALWAYS invoke `ppanel-migrate up` (idempotent — no-op if already at latest)
  3. Create admin if not present
  4. Continue with redis/queue/scheduler

Changes:
  - Add tools/migrate/ — Go module wrapping golang-migrate
    * migrate/migrate.go: Migrate() + RunUp() (RunUp bypasses the iofs vs
      os.ErrNotExist sentinel mismatch that breaks Up() on already-migrated DBs)
    * migrate/sql/{postgres,mysql}/: SQL files copied from server
    * cmd/migrate/main.go: CLI (up/down/version/force/drop)
  - Widen password column from varchar(100) → varchar(255) in
    00001_init_schema.up.sql to fit Rust's 112-char PBKDF2 hash
  - Rewrite src/migration.rs:
    * Remove sqlx::migrate!, run_migrations(), schema_present()
    * Add ensure_schema(cfg): unconditionally invokes ppanel-migrate up
  - Fix src/db.rs::build_dsn: previously used cfg.config verbatim even when
    the default config string was for the wrong dialect (PG got MySQL's
    charset=utf8mb4 string and crashed on connect)
  - Delete old migrations/{postgres,mysql}/ (no longer used)

Co-Authored-By: Claude <noreply@anthropic.com>

tools/migrate/migrate/sql/mysql/02117_site_custom_data.up.sql

@@ -0,0 +1,7 @@
+INSERT INTO `system` (`category`, `key`, `value`, `type`, `desc`, `created_at`, `updated_at`)
+SELECT 'site', 'CustomData', '{
+  "kr_website_id": ""
+}', 'string', 'Custom Data', '2025-04-22 14:25:16.637', '2025-10-14 15:47:19.187'
+    WHERE NOT EXISTS (
+    SELECT 1 FROM `system` WHERE `category` = 'site' AND `key` = 'CustomData'
+);