Config: drop unused search_backend knob

Nothing read it; services/search.py already picks SQLite vs Postgres by
dialect. The 'postgres_fts' default was a leftover from the server-era
config and only invited confusion.

backend/shonar/core/config.py

@@ -90,8 +90,9 @@ class Settings(BaseSettings):
     llm_base_url: str = ""
     llm_api_key: str = Field(default="", repr=False)
 
-    # --- Search ---------------------------------------------------------
-    search_backend: str = "postgres_fts"  # postgres_fts (meilisearch: TODO)
+    # Search needs no setting: services/search.py picks its path by SQL
+    # dialect (SQLite substring scan / Postgres tsvector). A Meilisearch or
+    # OpenSearch backend would replace that module, not add a config knob.
 
     # --- Retention --------------------------------------------------------
     # Grace window before the sweep hard-deletes soft-deleted recordings