feat: add deepsearch research system with CLI prune/clear and database schema

Implement a multi-agent deep web research subsystem including CLI commands for pruning expired jobs and clearing all artifacts, database tables for sessions/messages/URL cache with indexes, config paths for chroma storage, and internal embed URL for vector operations.

devplacepy/services/openai_gateway/service.py

@@ -89,6 +89,16 @@ class GatewayService(BaseService):
             help="Max concurrent upstream forwards per worker (connection pool + semaphore).",
             group="Upstream",
         ),
+        ConfigField(
+            "gateway_system_preamble",
+            "System preamble",
+            type="text",
+            default=config.SYSTEM_PREAMBLE_DEFAULT,
+            help="Operator text prepended ahead of every chat request's system message "
+            "(before an auto-injected EU-format date line and the client's own system "
+            "content, all in one system message). Leave blank to disable.",
+            group="Prompt",
+        ),
         ConfigField(
             "gateway_vision_enabled",
             "Vision augmentation",