SIENTIAPDE-1712

Add unique constraint on (model_id, timestamp, variable) in schema and implement idempotent export test to ensure no duplicates are created. Update CoreScouter and related tests to handle conflict resolution by ignoring duplicates.

e2e/conftest.py

@@ -89,6 +89,7 @@ def _create_schema_and_table(engine):
                 value numeric NULL,
                 "timestamp" timestamptz NOT NULL,
                 created_at timestamptz DEFAULT CURRENT_TIMESTAMP NOT NULL,
+                CONSTRAINT unique_timestamp_variable UNIQUE (model_id, "timestamp", variable),
                 PRIMARY KEY (id, created_at)
             );
         """