feat(event-file): implement atomic V3 upload action for event files

- Creates api_v3_actions_event_file.py with a specialized /upload endpoint.
- Handles physical storage (hosted_file), generic linking, and event association (event_file) in one request.
- Implements intelligent ID resolution to prevent duplicate event associations for the same physical file.
- Updates documentation in GUIDE__V3_FRONTEND_API.md.

documentation/GUIDE__V3_FRONTEND_API.md

@@ -107,7 +107,35 @@ V3 uses specialized **"Action"** routes for binary operations to separate proces
 
 ---
 
-## 5. Hosted File Management (Legacy)
+## 5. Event File Management (Specialized V3 Actions)
+
+While `hosted_file` handles generic storage, `event_file` actions are context-aware and atomic for the Event module.
+
+### A. Atomic Upload Action
+**Path**: `POST /v3/action/event_file/upload`
+**Format**: `multipart/form-data`
+
+| Field | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| `file_list` | File[] | Yes | Files to upload. |
+| `account_id`| String | Yes | Owner account. |
+| `for_type`  | String | Yes | Parent object type (e.g., `event_session`). |
+| `for_id`    | String | Yes | Random ID of the parent object. |
+| `event_id`  | String | No | Optional event context. |
+| `title`     | String | No | Display title for the file. |
+
+**Features:**
+- **Atomic Creation:** Automatically creates the `hosted_file`, the `hosted_file_link`, AND the `event_file` association in one request.
+- **Intelligent Updates:** If the same file is uploaded again for the same object, it updates the metadata instead of creating a duplicate association.
+- **Enriched Return:** Returns full `event_file` objects with nested `hosted_file` data.
+
+### B. Specialized Download
+**Path**: `GET /v3/action/event_file/{id}/download`
+*Semantic alias for the universal hosted_file downloader.*
+
+---
+
+## 6. Hosted File Management (Legacy)
 
 The following endpoints are maintained for backward compatibility but should be migrated to V3 Actions.