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. Probe for the `user` table (schema marker)
  3. If missing, fork-exec `ppanel-migrate up` to bring schema to v2131
  4. Create admin if not present
  5. Continue with redis/queue/scheduler

Verified:
  - Empty DB → ppanel-migrate applies all 39 migrations in ~0.4s, admin created
  - Go server's ppanel_go DB → schema detected, migrate skipped, clean start

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
    (Rust uses 16-byte salt; Go's 8-byte salt fits in 100 but Rust's doesn't)
  - Rewrite src/migration.rs:
    * Remove sqlx::migrate!, run_migrations()
    * Add ensure_schema(db, cfg): probe + invoke ppanel-migrate subprocess
  - 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/go.mod

@@ -0,0 +1,11 @@
+module github.com/perfect-panel/ppanel-backend/tools/migrate
+
+go 1.25.0
+
+require github.com/golang-migrate/migrate/v4 v4.19.1
+
+require (
+	filippo.io/edwards25519 v1.1.0 // indirect
+	github.com/go-sql-driver/mysql v1.8.1 // indirect
+	github.com/lib/pq v1.10.9 // indirect
+)