fix(v3-actions): implement from_hosted_file and harden vision IDs

1. Implemented specialized 'from_hosted_file' action for Event Files.\n2. Fixed ValueError in Pydantic models by removing default/default_factory conflict.\n3. Hardened integer stripping to strictly enforce Vision Standards.\n4. Updated documentation for the new action route.

documentation/GUIDE__V3_FRONTEND_API.md

@@ -151,6 +151,23 @@ While `hosted_file` handles generic storage, `event_file` actions are context-aw
 **Path**: `GET /v3/action/event_file/{id}/download`
 *Semantic alias for the universal hosted_file downloader.*
 
+### C. Link Existing Hosted File
+**Path**: `POST /v3/action/event_file/from_hosted_file/{hosted_file_id}`
+
+Use this when a file is already in standard storage (e.g., from a previous session or general archive) and you want to link it to an Event object without re-uploading.
+
+| Parameter | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| `hosted_file_id` | Path | Yes | String ID of the physical file. |
+| `for_type` | Body | Yes | Target object type (e.g., `event_session`). |
+| `for_id` | Body | Yes | String ID of the target object. |
+| `event_id` | Body | No | Optional event context. |
+
+**Features:**
+- **Vision IDs:** Accepts string IDs in both the path and JSON body.
+- **Relational Integrity:** Automatically creates both `hosted_file_link` and `event_file` records.
+- **Enriched Return:** Returns the full `event_file` object.
+
 ---
 
 ## 6. Hosted File Management (Legacy)