feat: add extraBody support to model aliases

- Add extra_body column to model_aliases table (SQLite text, PG jsonb)
- Add extraBody to ModelConfigSchema (z.record, optional)
- Read/write extraBody in config-repository (rowToModelConfig, saveAlias)
- Merge alias-level extraBody in dispatcher (chat, embeddings, speech, image)
  with highest priority (overrides provider and model-level extraBody)
- Add AliasExtraBodyEditor component in the Advanced disclosure
- Add extraBody to frontend Alias type, saveAlias, and getAliases
- Update test to handle pending migration column

Author: mcowger

packages/backend/drizzle/migrations/0045_add_alias_extra_body.sql

@@ -0,0 +1 @@
+ALTER TABLE `model_aliases` ADD `extra_body` text;