feat: add POST /vouching/request endpoint for learners

[EmeditWeb]

Problem

Learners have no way to request a vouch from
a specific mentor. The vouching flow is one-sided.

What To Build

Add POST /api/v1/vouching/request endpoint.
Requires JWT auth (learner wallet).

Body:
{
mentorAddress: string (valid Stellar address)
}

Creates a vouch request record in Supabase
vouch_requests table.

Sends notification to mentor (if push token exists).

Files To Touch

• src/modules/vouching/vouching.controller.ts
• src/modules/vouching/vouching.service.ts
• supabase/migrations/[timestamp]_vouch_requests.sql

Acceptance Criteria

• Creates vouch request in database
• Validates mentor address format
• Prevents duplicate requests to same mentor
• Returns 409 if request already exists
• Full Swagger docs

Mandatory Checks Before PR

• npm run build passes
• Migration file created
• PR references this issue

[Iwayemi-Kehinde]

I would like to work on this issue as I have reviewed the scope, I should have a PR Ready in 50 minutes.