Harden AI writer response handling

db/migrations/021_writer_contract_no_empty_response.sql

@@ -0,0 +1,19 @@
+UPDATE app_settings
+SET value_json = to_jsonb($contract$
+OUTPUT SCHEMA (return array matching input order):
+{"rewrites":[{"id":123,"category_id":2,"text":"готовый текст без хэштегов","notes":"короткая заметка для редактора или null"}]}
+
+Rules:
+- Input is a JSON object with key "posts" containing accepted posts.
+- Input has key "categories" with objects: id, name, tag.
+- Each post includes producer_name and producer_tag. producer_name can be a manufacturer, shop, or publishing source.
+- Pick exactly one category from the categories list in input and return its numeric id as category_id.
+- Do not add links or hashtags to rewritten text.
+- Return JSON only. No markdown. No text outside JSON.
+- Never return an empty object. Never omit the "rewrites" key.
+- Return one rewrite for every input post id.
+- The "text" field must be a ready-to-publish Russian post with normal paragraph line breaks encoded as JSON string newlines.
+- The "notes" field must be a short editor note in Russian or null.
+$contract$::text),
+updated_at = NOW()
+WHERE key='ai_writer_contract';