feat: preview-DB seeding + reset endpoint with deterministic results (bounty #10)

[Kasuki354]

Summary

Implements bounty #10: Preview-DB seeding + reset endpoint

Invariant

Two consecutive probe runs against a fresh preview give identical results.

Implementation

POST /admin/seed:

• Wipes all existing data (votes, options, polls, clones) in a transaction
• Creates deterministic seed: 1 clone (poll), 1 poll (Best framework?), 3 options (Fastify/Express/Hono), 3 votes
• Idempotent: running twice produces identical state
• Optional auth via x-admin-token header (only enforced if ADMIN_TOKEN env var is set)

POST /admin/reset:

• Wipes all data in a transaction
• Returns empty state

Both endpoints:

• Use prisma. for atomicity
• Log events (db.seeded, db.reset) for observability
• Return clean JSON status

Tests:

• Seed creates deterministic data (1 clone, 1 poll, 3 options, 3 votes)
• Two consecutive seed+probe runs give identical results
• Reset clears all data
• Seed after reset works
• Seed is idempotent (same state after running twice)
• No regression: poll creation and voting still work after seeding

Resolves #10