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)]

crates/presentation/src/handlers/feed.rs

@@ -133,6 +133,20 @@ pub async fn user_thoughts_handler(
     })))
 }
 
+pub async fn get_popular_tags(
+    State(s): State<AppState>,
+    Query(params): Query<std::collections::HashMap<String, String>>,
+) -> Result<Json<serde_json::Value>, ApiError> {
+    let limit: usize = params.get("limit").and_then(|v| v.parse().ok()).unwrap_or(20);
+    let tags = s.tags.popular_tags(limit.min(100)).await?;
+    Ok(Json(serde_json::json!({
+        "tags": tags.iter().map(|(name, count)| serde_json::json!({
+            "name": name,
+            "thought_count": count,
+        })).collect::<Vec<_>>()
+    })))
+}
+
 #[utoipa::path(
     get, path = "/tags/{name}",
     params(