Add per-service system prompts with AI Settings tab view

Each feature service owns its system prompt in its config JSON on the
shared volume. The AI Settings page now has General and System Prompts
tabs — admins can view and edit any service's prompts at runtime with
changes taking effect within 30 s (config cache TTL).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

features/doc-service/STATUS.md

@@ -72,6 +72,8 @@ categories    many-to-many via category_assignments
 
 ### AI extraction (via ai-service)
 
+System prompt and user prompt template are loaded at runtime from `doc_service_config.json` (`system_prompts` key). Defaults are built into the service and used as fallback if the config key is absent. Changes made via the AI Settings UI take effect within 30 seconds (config cache TTL).
+
 Prompt sends the first 50 000 chars of extracted text. Expected JSON response includes:
 - `title` — suggested human-readable title
 - `document_type` — invoice / bill / receipt / order / expense / revenue / unknown