feat(paste): implement Paste clone with concurrency handling

[iQodeIT]

What this does

Closes #4. Implements a view-once Paste service with TTL and concurrency handling.

The invariant

The viewsAllowed counter is decremented atomically using a single Prisma update with a where clause (viewsLeft: { gt: 0 }). This ensures that even under high concurrent load, the paste is never served more times than allowed.

How I verified it

Ran pnpm -w run test and verified the new paste.test.ts which fires 20 concurrent reads against a 1-view paste; exactly 1 succeeded (200) and 19 failed (410).

Checklist

• Whole test suite is green from a clean checkout
• - [x] The concurrency/correctness probe passes with zero 5xx
• - [x] Migrations apply cleanly on a fresh DB; key events are logged
• - [x] No regressions to other clones' probes
• - [x] I've read TERMS.md
  AI usage (directed vs. decided):
  I directed the AI to implement the Paste clone based on the requirements in issue [bounty] Paste clone (view-once) #4, specifically focusing on the concurrency invariant. I decided on the specific Prisma update logic to ensure atomicity.