Add warnings about schema changes and remove ad-hoc index creation

- Remove ad-hoc index creation from database.go Initialize method
- Add clear comments to both database.go and schema.sql warning that
  ALL schema changes must be made in schema.sql only
- We do not support migrations, schema changes outside schema.sql are forbidden

internal/database/schema.sql

@@ -1,3 +1,7 @@
+-- IMPORTANT: This is the ONLY place where schema changes should be made.
+-- We do NOT support migrations. All schema changes MUST be in this file.
+-- DO NOT make schema changes anywhere else in the codebase.
+
 CREATE TABLE IF NOT EXISTS asns (
 	id TEXT PRIMARY KEY,
 	number INTEGER UNIQUE NOT NULL,