fix: guard SqlController Migrate() with IHistoryRepository.Exists()

[renemadsen]

Summary

Add IHistoryRepository.Exists() guard around Database.Migrate() in SqlController so DDL is only issued when actually needed.

Why

The existing GetPendingMigrations().Any() guard throws (and re-throws) if __EFMigrationsHistory doesn't exist yet, crashing startup on a fresh SDK DB. With the IHistoryRepository.Exists() outer guard:

• Fresh install: Exists() = false → falls through to Migrate() → creates schema ✅
• Schema current (production): Exists() = true, GetPendingMigrations() = empty → skips entirely, no DDL, no GET_LOCK ✅
• Pending migrations: Exists() = true, pending found → Migrate() applies them ✅

On a Galera cluster with ~250 tenants, this eliminates SDK DDL from every pod restart.

Test Plan

• Fresh SDK DB: migrations run and schema is created
• Existing SDK DB (schema current): no DDL on restart

🤖 Generated with Claude Code

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