Add ContestRequest model for Contest Creator rights request workflow

[GauriGupta21]

What this adds

A new ContestRequest model representing a user's request for Contest Creator rights — this is a permission request, not a contest itself.

Why this design

Initially I migrated the old wikicontest backend's ContestRequest model, which duplicated almost all Contest fields (name, dates, scoring, categories, jury members, etc.) via a shared mixin.

After confirming the actual product workflow with the team, we agreed this was unnecessary:

• Requesting Contest Creator rights only needs a reason + Wikimedia edit count (300+ eligibility) + review status.
• Full contest details are only collected later, through a separate Create Contest form, once rights are granted.
• Keeping ContestRequest minimal avoids duplicating Contest's schema and keeps the two concerns (permission vs. contest) properly decoupled.

Fields

• id
• user_id (FK → users.id — requester)
• reason (required)
• edit_count (cached Wikimedia edit count at request time)
• status (pending / approved / rejected, default 'pending')
• reviewed_by (FK → users.id — superadmin who reviewed)
• reviewed_at
• rejection_reason
• created_at

How I tested this

1. Generated and reviewed the Alembic migration manually (no blind autogenerate trust)
2. Applied migration locally (flask db upgrade) against SQLite
3. Verified via flask shell:

   from model import db, User, ContestRequest
   u = User(username="TestUser456")
   db.session.add(u); db.session.commit()

   cr = ContestRequest(user_id=u.id, reason="I have 350+ edits and want to run a contest for my wikiproject")
   db.session.add(cr); db.session.commit()

   print(cr.id, cr.status, cr.reason, cr.user_id, cr.edit_count)
   # Output: 1 pending "I have 350+ edits..." 1 None

Confirms: record saves, default status='pending' applies automatically, all fields retrieve correctly.