feat(storage): add get_observations_with_empty_metadata query

NULL-safe query for observations with empty facts, concepts, and keywords. Used by backfill-metadata CLI command.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Author: Stranmor

crates/storage/src/pg_storage/observations.rs

@@ -402,4 +402,25 @@ impl ObservationStore for PgStorage {
         }
         Ok(())
     }
+
+    async fn get_observations_with_empty_metadata(
+        &self,
+        limit: usize,
+    ) -> Result<Vec<Observation>, StorageError> {
+        let rows = sqlx::query(&format!(
+            "SELECT {} \
+             FROM observations \
+             WHERE (facts IS NULL OR jsonb_array_length(facts) = 0) \
+               AND (concepts IS NULL OR jsonb_array_length(concepts) = 0) \
+               AND (keywords IS NULL OR jsonb_array_length(keywords) = 0) \
+             ORDER BY created_at DESC, id DESC LIMIT $1",
+            super::OBSERVATION_COLUMNS
+        ))
+        .bind(usize_to_i64(limit))
+        .fetch_all(&self.pool)
+        .await?;
+        Ok(collect_skipping_corrupt(
+            rows.iter().map(row_to_observation),
+        ))
+    }
 }

crates/storage/src/traits/observation.rs

@@ -75,4 +75,11 @@ pub trait ObservationStore: Send + Sync {
         id: &str,
         metadata: &ObservationMetadata,
     ) -> Result<(), StorageError>;
+
+    /// Get observations that have empty metadata (facts, concepts, keywords all empty).
+    /// Used by the backfill-metadata CLI command to find observations needing enrichment.
+    async fn get_observations_with_empty_metadata(
+        &self,
+        limit: usize,
+    ) -> Result<Vec<Observation>, StorageError>;
 }