POST /boards/{id}/acl returns 500 instead of 409 when participant already in ACL

Steps to reproduce

1. Share a board with a user (e.g. POST /index.php/apps/deck/api/v1.0/boards/{id}/acl with {"type":0,"participant":"alice","permissionEdit":true,"permissionShare":true,"permissionManage":true}).
2. Call the same endpoint a second time with the same payload.

Expected behaviour

A 4xx client error (409 Conflict, or 400 Bad Request) indicating that the ACL entry already exists. Clients can handle this idempotently.

Actual behaviour

HTTP 500 Internal Server Error. Response body:

{
  "status": 500,
  "message": "Internal server error: Please contact the server administrator if this error reappears multiple times, please include the request ID \"XXXX\" below in your report.",
  "requestId": "XXXX"
}

Server logs likely contain a UNIQUE constraint failed on oc_deck_board_acl (user+board pair).

Why this matters

Many client integrations call shareBoard idempotently on startup/initialize. Receiving a 500 instead of a 4xx forces clients to either (a) pre-fetch the full ACL and diff it before every share call, or (b) treat 500s as potentially-retryable, which masks real server errors. A proper 4xx lets clients write cleaner code.

Suggested fix

In the ACL controller, catch the unique-constraint violation and return 409 (or 400). The existing check could be simplified to a pre-read-and-skip or an INSERT ... ON CONFLICT pattern.

Server configuration

• Operating system: Hosted (STRATO HiDrive/storageshare)
• Web server: Nginx
• Database: PostgreSQL (presumed — hosted)
• PHP version: 8.x
• Nextcloud version: 32.0.6
• Deck version: 1.16.3
• Updated from an older Nextcloud/Deck or fresh install: long-running install

Client configuration

• Browser: N/A (API client)
• Operating system: Linux

Reproduction command

curl -u USER:PASS -H "OCS-APIRequest: true" -H "Content-Type: application/json" \
  -X POST "https://NC_HOST/index.php/apps/deck/api/v1.0/boards/{BOARD_ID}/acl" \
  -d '{"type":0,"participant":"EXISTING_USER","permissionEdit":true,"permissionShare":true,"permissionManage":true}'
# → HTTP 500

Inspect the ACL controller handling POST /index.php/apps/deck/api/v1.0/boards/{id}/acl, then reproduce the duplicate-participant request with the supplied curl command. Done means a second request for the same participant returns a 409 or 400 client error instead of HTTP 500, with coverage for that behavior.