✨ feat: implement new feature for enhanced user experience

.agents/skills/content-authoring/SKILL.md

@@ -344,7 +344,6 @@ Edit `types/global.d.ts`:
 ```typescript
 interface MyCollectionEntry {
     id?: string
-    _id?: string
     active?: boolean
     name?: string
     // ... fields matching your YAML

.agents/skills/frontend-architecture/SKILL.md

@@ -292,6 +292,31 @@ const result = await postDBEntry("content", { name: "New Page", active: true })
 const { data, count } = await api<MyType[]>("mycollection", { filter: { active: true }, limit: 20 })
 ```
 
+### `aggregate` for sub-queries
+
+The server supports an `aggregate` parameter to compute reverse aggregates against another collection and store the result under `_aggregate`. This efficiently calculates counts, sums, existence, etc. without embedding the target documents.
+
+```typescript
+const res = await api<MyEntry[]>("mycollection", {
+    filter: { active: true },
+    params: {
+        // String syntax: "collection:foreignField:op:valueField:as"
+        aggregate: "posts:categoryId:count",
+        // JSON syntax for advanced use cases (custom source field, filtering)
+        aggregate: JSON.stringify({
+            collection: "comments",
+            foreignField: "entryId",
+            op: "count",
+            filter: { approved: true },
+            as: "approvedComments",
+        }),
+    },
+})
+// Result in res.data[0]._aggregate.postsCount  and res.data[0]._aggregate.approvedComments
+```
+
+Available operations: `count` (default), `exists`, `sum`, `avg`, `min`, `max`.
+
 ### Error handling
 
 ```typescript
@@ -311,6 +336,31 @@ try {
 }
 ```
 
+### `aggregate` for sub-queries
+
+The server supports an `aggregate` parameter to compute reverse aggregates against another collection and store the result under `_aggregate`. This efficiently calculates counts, sums, existence, etc. without embedding the target documents.
+
+```typescript
+const res = await api<MyEntry[]>("mycollection", {
+    filter: { active: true },
+    params: {
+        // String syntax: "collection:foreignField:op:valueField:as"
+        aggregate: "posts:categoryId:count",
+        // JSON syntax for advanced use cases (custom source field, filtering)
+        aggregate: JSON.stringify({
+            collection: "comments",
+            foreignField: "entryId",
+            op: "count",
+            filter: { approved: true },
+            as: "approvedComments",
+        }),
+    },
+})
+// Result in res.data[0]._aggregate.postsCount  and res.data[0]._aggregate.approvedComments
+```
+
+Available operations: `count` (default), `exists`, `sum`, `avg`, `min`, `max`.
+
 ### Error handling guidelines
 
 | Scenario                          | Approach                                          |
@@ -426,7 +476,7 @@ Nicht aufgeführte Felder (z.B. `description`, `specs`) entfallen – spart Band
 - **Never `spaNavigate()` in SSR** — always guard with `typeof window !== "undefined"`.
 - **Store subscriptions in modules** — if subscribing to stores outside components, remember to unsubscribe to prevent memory leaks.
 - **API PUT returns only changed fields** — don't expect a full object back from PUT requests.
-- **`_id` not `id` for filters** — API filters use MongoDB's `_id`, but response objects may have both `id` and `_id`.
+- **`_id` not `id` for filters** — API filters use MongoDB's `_id`, but response objects only have `id` as string via API.
 - **`$location` strips trailing slashes** — `/about/` becomes `/about` (except root `/`).
 - **Content cache is 1 hour** — `getCachedEntries` caches in memory for 1h. For admin previews, use `getDBEntries` (uncached).
 - **`$effect` alone is not SSR** — server-side rendering must trigger the same data path explicitly outside browser-only reactive effects.