fix: use EXECUTE for dynamic SQL in migrations referencing old schema

PL/pgSQL pre-validates column/table references in static SQL statements
inside DO blocks before evaluating IF/RETURN guards. This caused
migrations 0010-0061 to fail on fresh databases where the canonical
schema uses `providers`/`provider_id` instead of `llm_providers`/
`llm_provider_id`.

Wrap all SQL that references potentially non-existent old schema objects
(llm_providers, llm_provider_id, tts_providers, tts_models, etc.) in
EXECUTE strings so they are only parsed at runtime when actually reached.

db/migrations/0042_provider_enable.up.sql

@@ -5,6 +5,6 @@
 DO $$
 BEGIN
   IF EXISTS (SELECT 1 FROM information_schema.tables WHERE table_name = 'llm_providers') THEN
-    ALTER TABLE llm_providers ADD COLUMN IF NOT EXISTS enable BOOLEAN NOT NULL DEFAULT true;
+    EXECUTE 'ALTER TABLE llm_providers ADD COLUMN IF NOT EXISTS enable BOOLEAN NOT NULL DEFAULT true';
   END IF;
 END $$;