fix(download): strict ID typing — remove cross-resolution from hosted_file download

event_file download now resolves event_file_id → hosted_file_id explicitly before
delegating, rather than relying on a cross-resolution fallback inside the hosted_file
endpoint. The hosted_file download endpoint now only accepts hosted_file IDs.

Cross-resolution was added reactively (ea117bf) to patch incorrect frontend ID usage
and was never a deliberate design decision. With no per-record account ownership check
on the download path, the implicit ID aliasing was an unauditable gap.

- download_event_file_action: resolves event_file → hosted_file via Redis + SQL before
  delegating; 404s explicitly if chain is broken
- download_file_action: strict hosted_file ID only; cross-resolution fallback removed
- Also fixes ?key= not being forwarded (was missing from event_file endpoint signature)
- TODO: per-record account ownership check (P1), archive_content download endpoint (P2)
- Docs: breaking change note added to frontend guide (remove ~2026-06-24)

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

documentation/GUIDE__AE_API_V3_for_Frontend.md

@@ -286,7 +286,7 @@ When seeding new lookup data (e.g., adding timezones in bulk):
 
 ## 5. Event File Data Retrieval (Hosted Files)
 
-Every Event File (`event_file`) **must** have a linked Hosted File (`hosted_file`). The Hosted File itself is a metadata record for binary content (files), which is accessed via separate Action endpoints (e.g., `/v3/action/hosted_file/download`). This API endpoint provides metadata about the associated hosted file. To retrieve this additional metadata:
+Every Event File (`event_file`) **must** have a linked Hosted File (`hosted_file`). The Hosted File is a metadata record for binary content, accessed via dedicated Action endpoints. To download an event file use `/v3/action/event_file/{event_file_id}/download` — not the hosted_file endpoint directly (each endpoint only accepts its own ID type). To retrieve hosted file metadata alongside an event file record:
 
 *   **Endpoint:** `GET /v3/crud/event_file/{event_file_id}`
 *   **Query Parameter:** Add `inc_hosted_file=true`
@@ -308,7 +308,8 @@ Event files can be downloaded without standard auth headers using one of two byp
 - **Method:** `GET`
 - **Path:** `/v3/action/event_file/{event_file_id}/download`
 
-The endpoint also accepts `hosted_file_id` or `archive_content_id` directly — it resolves the chain automatically.
+> [!WARNING]
+> **Breaking change (2026-06-10):** This endpoint now requires an `event_file_id`. Previously it accepted `hosted_file_id` or `archive_content_id` and resolved the chain automatically — that cross-resolution has been removed. Pass the correct ID type for the endpoint you are calling. If you were routing downloads through `/v3/action/hosted_file/{hosted_file_id}/download` as a workaround, switch to this endpoint using `event_file_id`. *(Remove this note after ~2026-06-24.)*
 
 #### Auth bypass options