Add admin LLM model setting; rename Pi to LLM in the UI; drop table Review column

- New server/llm.js consolidates ModelRuntime + model selection: admin
  setting (app_settings.llm_model) > LLM_MODEL/PREFILL_MODEL env > first
  available; used by both prefill and roast evaluation
- GET/PUT /api/admin/llm lists configured models and stores the choice
  (validated against the list; empty = auto; audited); admin page gains
  an LLM section with a model picker
- All user-facing 'Pi agent' wording is now 'LLM'; no_model error message
  no longer references the pi CLI
- /roasts table: Review column removed (review lives in the detail view)

Co-Authored-By: Claude Fable 5 <[email protected]>

db/migrations/005_llm_setting.sql

@@ -0,0 +1,3 @@
+-- Additive only. Admin-selectable LLM model ("provider:id") for prefill and roast reviews;
+-- empty means auto (first available configured model).
+INSERT INTO app_settings(key,value) VALUES ('llm_model','');