feat: add convert_file endpoint to v3 actions hosted_file router

Exposes GET /v3/action/hosted_file/{id}/convert_file using AccountContext
(v3 auth pattern) alongside the legacy /hosted_file/ route. Accepts
link_to_type, link_to_id, filename_no_ext, and to_type query params.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routers/api_v3_actions_hosted_file.py

@@ -17,6 +17,7 @@ from app.methods.hosted_file_methods import (
     create_hosted_file_obj, load_hosted_file_obj, save_file,
     create_hosted_file_link, delete_hosted_file_link, get_hosted_file_link_rec_list
 )
+from app.methods.lib_media import convert_file_method
 from app.lib_general_v3 import (
     AccountContext, get_account_context, get_account_context_optional,
     SerializationParams, DelayParams
@@ -447,3 +448,36 @@ async def delete_file_action(
         "record_removed": record_removed,
         "method": method
     }, status_message="Deletion process complete.")
+
+
+# ### BEGIN ### API V3 Hosted File Action ### convert_file() ###
+@router.get('/{hosted_file_id}/convert_file', response_model=Resp_Body_Base)
+async def convert_file(
+        hosted_file_id: str = Path(min_length=11, max_length=22),
+        link_to_type: str = Query(...),
+        link_to_id: str = Query(...),
+        filename_no_ext: str = Query('automated_hosted_file_conversion'),
+        to_type: str = Query('webp'),
+        account: AccountContext = Depends(get_account_context),
+        ):
+    """
+    Convert a hosted file to another format (e.g. PDF → webp image).
+    Runs pdf2image server-side and saves the result as a new hosted_file record
+    linked to the same parent object via link_to_type / link_to_id.
+    """
+    lid_int = redis_lookup_id_random(record_id_random=link_to_id, table_name=link_to_type)
+    if not lid_int:
+        raise HTTPException(status_code=404, detail=f"Linked object not found: {link_to_type}:{link_to_id}")
+    result = await convert_file_method(
+        hosted_file_id=hosted_file_id,
+        link_to_type=link_to_type,
+        link_to_id=lid_int,
+        account_id=account.account_id,
+        account_id_random=account.account_id_random,
+        filename_no_ext=filename_no_ext,
+        to_type=to_type
+    )
+    if result:
+        return mk_resp(data=result)
+    return mk_resp(data=None, status_code=400, status_message="Conversion failed.")
+# ### END ### API V3 Hosted File Action ### convert_file() ###