feat: GET /tags/popular — top tags by usage count

crates/adapters/postgres/src/tag.rs

@@ -51,6 +51,21 @@ impl TagRepository for PgTagRepository {
 
         Ok(Paginated { items: rows.into_iter().map(Thought::from).collect(), total, page: page.page, per_page: page.per_page })
     }
+
+    async fn popular_tags(&self, limit: usize) -> Result<Vec<(String, i64)>, DomainError> {
+        sqlx::query_as::<_, (String, i64)>(
+            "SELECT t.name, COUNT(tt.thought_id) AS thought_count
+             FROM tags t
+             JOIN thought_tags tt ON t.id = tt.tag_id
+             GROUP BY t.id, t.name
+             ORDER BY thought_count DESC
+             LIMIT $1"
+        )
+        .bind(limit as i64)
+        .fetch_all(&self.pool)
+        .await
+        .map_err(|e| DomainError::Internal(e.to_string()))
+    }
 }
 
 #[cfg(test)]