Fix database schema migration for missing columns

[railway-app]

Problem

The application fails to start against older PostgreSQL databases because GORM's AutoMigrate introspects existing tables before adding new columns. When those tables are missing columns the current code expects (prefill_groups.type, tokens.type, tokens.models, channels.deleted_at), the introspection queries themselves fail with "column does not exist" errors, preventing the migration from completing and leaving the service returning 502s.

Solution

Added model/schema_repair.go with a repairSchema() function that explicitly adds each missing column via ALTER TABLE … ADD COLUMN before AutoMigrate runs. Each operation is guarded by an existence check (information_schema.columns for Postgres/MySQL, PRAGMA table_info for SQLite) so it is fully idempotent and safe on every startup. repairSchema() is called at the top of migrateDB() in model/main.go, covering all three supported database backends.

Changes

• Created model/schema_repair.go
• Modified model/main.go

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