Fix schema-sync indexes for non-default PostgreSQL schemas (#3084)

Indexes generated from `#[sea_orm(indexed)]` and `#[sea_orm(unique_key = "...")]` on an entity with a `schema_name` are now created against the schema-qualified table on PostgreSQL (e.g. `ON "sys"."app_user"` instead of `ON "app_user"`), fixing schema-sync failures (`relation "..." does not exist`) and wrong-schema targeting. Entities without a `schema_name` are unaffected.

MySQL and SQLite keep using unqualified index targets — SeaQuery's index builders for those backends panic on a schema-qualified reference. Routing both index-generation paths through a shared backend-aware helper also fixes a latent panic when syncing a `#[sea_orm(unique)]` column on a `schema_name` entity against MySQL/SQLite.

Closes #3084.

Co-authored-by: oywm <2656771275@qq.com>