platform: Guard seed script and untrack dev database file

[jeromehardaway]

Intent

The seed script cannot wipe a non-dev database, and the repo stops tracking a local SQLite file.

Scope

• Env guard in prisma/seed.ts (refuse to wipe when DATABASE_URL is not a local/dev database).
• Untrack prisma/dev.db.
• Fix stale SQLite hints in .env.example (coordinates with Audit .env.example vs .env — document every required env var #1063).

Acceptance criteria

• Seed refuses on a production URL
• dev.db untracked
• Docs aligned

Dependencies

Relates to #1063.

Touches: Data model.