phsl / new-api

Replace legacy single-column unique indexes with composite unique indexes on
(name, deleted_at) and introduce a safe index drop utility to eliminate
duplicate-key errors and noisy MySQL 1091 warnings.

WHAT
• model/model_meta.go
  - Model.ModelName  → `uniqueIndex:uk_model_name,priority:1`
  - Model.DeletedAt  → `index; uniqueIndex:uk_model_name,priority:2`
• model/vendor_meta.go
  - Vendor.Name      → `uniqueIndex:uk_vendor_name,priority:1`
  - Vendor.DeletedAt → `index; uniqueIndex:uk_vendor_name,priority:2`
• model/main.go
  - Add `dropIndexIfExists(table, index)`:
    • Checks `information_schema.statistics`
    • Drops index only when present (avoids Error 1091)
  - Invoke helper in `migrateDB` & `migrateDBFast`
  - Remove direct `ALTER TABLE … DROP INDEX …` calls

WHY
• Users received `Error 1062 (23000)` when re-creating a soft-deleted
  model/vendor because the old unique index enforced uniqueness on name alone.
• Directly dropping nonexistent indexes caused MySQL `Error 1091` noise.

HOW
• Composite unique indexes `(model_name, deleted_at)` / `(name, deleted_at)`
  respect GORM soft deletes.
• Safe helper ensures idempotent migrations across environments.

RESULT
• Users can now delete and re-add the same model or vendor without manual SQL.
• Startup migration runs quietly across MySQL, PostgreSQL, and SQLite.
• No behavior changes for existing data beyond index updates.

TEST
1. Add model “deepseek-chat” → delete (soft) → re-add → success.
2. Add vendor “DeepSeek”     → delete (soft) → re-add → success.
3. Restart service twice → no duplicate key or 1091 errors.