fix(applications): allow re-applying after withdrawing

[clawedassistant26]

Problem

A withdrawn application still matched the "already applied" duplicate check in POST /api/applications, so an applicant who withdrew could never re-apply to that gig (You have already applied to this gig). With no active application, the poster has nothing to accept, so POST /api/gigs/{id}/invoice stays permanently gated on Application must be accepted before creating an invoice. Net effect: completed, merged work can never be invoiced once an application has been withdrawn.

Hit in practice on a live gig where a withdrawn application dead-ended both re-apply and invoicing.

Fix

• Scope the duplicate check to non-withdrawn applications, so a withdrawn one no longer blocks re-applying.
• On resubmission, reuse the existing row (reset status to pending with the new cover letter / rate / etc.) instead of inserting. The UNIQUE(gig_id, applicant_id) constraint forbids a second row, so an insert would fail; an in-place update is correct.
• Downstream notification / email / webhook behaviour is unchanged.

Tests

Added src/app/api/applications/route.test.ts:

• regression: a withdrawn application is re-activated (201) via UPDATE, never a duplicate INSERT, with status reset to pending. Fails before this change (400), passes after.
• a still-active (pending) application continues to block re-applying (400).

Full suite green locally: vitest run -> 189 files / 1714 tests passed; tsc --noEmit clean; eslint clean on changed files.