[API] PUT /api/settings should validate required fields or support PATCH for partial updates

[imparpaulo01]

Problem

When calling PUT /api/settings with a partial payload (e.g., only updating SMTP config), the API accepts the request and replaces ALL settings with the partial payload. This causes:

1. Required fields like app.lang and upload.provider become empty strings
2. Listmonk crashes on next restart with panic: invalid value: app.lang=
3. Server becomes unbootable until database is manually fixed via direct SQL

Steps to Reproduce

1. Have a working Listmonk instance with configured settings
2. Call PUT /api/settings with only SMTP configuration:

{
  "smtp": [{...smtp config...}]
}

3. API returns success
4. Restart Listmonk container
5. Result: Server crashes with panic: invalid value: app.lang=

Expected Behavior

Either:

• Option A: Validate that required fields are present before accepting the update, return 400 error if critical settings would be wiped
• Option B: Support PATCH /api/settings for partial/merge updates (only modify fields that are sent)
• Option C: Document clearly that PUT replaces ALL settings and partial payloads will break the instance

Current Workaround

Must fetch ALL settings first via GET /api/settings, modify the specific field in the response, then PUT the entire object back. This is error-prone and not documented.

Environment

• Listmonk version: v6.0.0
• Deployment: Docker (via Coolify)
• Database: PostgreSQL 16

Related Issues

• Partially update a user #1681 (partial update painpoint for other endpoints)
• FEATURE: Swagger along Listmonk w/ better configuration and up-to-date definitions #1886 (swagger outdated for /api/settings)

Impact

This issue caused production downtime twice - the server became completely unbootable until we manually restored ~20 settings via direct database access.

[knadh]

Sorry to hear that @imparpaulo01. The PUT API doesn't support partial updates at all, unfortunately. This has been on the to-do for a long time, to add per-setting validation and also support partial updates. I hope to address it in a release this year.

[saurabhraghuvanshii]

Hi @knadh let me try on this !!