feat: create deterministic fixtures for prisma seed

[Opulencechuks]

Closes #126

Description:
This PR introduces deterministic, idempotent fixtures for the Prisma development seed dataset. It isolates mock user and module data into dedicated fixture files, assigns stable local-only IDs, and ensures that repeated seeding operations safely update existing records rather than duplicating them.

Changes:

• Created prisma/fixtures/users.ts containing local-only seed identities covering all expected test cases (verified learner, unverified learner, deactivated learner, admin, instructor, employer).
• Created prisma/fixtures/modules.ts isolating module data fixtures.
• Refactored prisma/seed.ts to rely on the new fixtures and implemented robust upsert operations for users, modules, completions, credentials, and transactions.
• Updated prisma/SETUP.md with instructions on seeding and explicitly resetting the dataset using npm run seed:reset.

Acceptance Criteria Met:

• Defined stable local-only IDs for learner, admin, instructor, and future employer.
• Seed verified, unverified, and disabled accounts.
• Seed current modules, completions, transactions, credentials.
• Make repeated seed runs idempotent or explicitly resettable.
• Exclude real addresses, secrets, and PII.
• Fixture identities are documented as local-only in SETUP.md.

Verification Evidence (Two-run seed and fixture verification):

Run 1 Output:

🌱 Starting database seed...
✅ Upserted 6 users (learners, employers, admin)
✅ Upserted 8 modules across all categories
✅ Upserted 12 completions
✅ Upserted 8 credentials
✅ Upserted 15 transactions
✅ Upserted webhook endpoint
🎉 Seed completed successfully
📊 Totals => users: 6, modules: 8, completions: 12, credentials: 8, transactions: 15

Run 2 Output:

🌱 Starting database seed...
✅ Upserted 6 users (learners, employers, admin)
✅ Upserted 8 modules across all categories
✅ Upserted 12 completions
✅ Upserted 8 credentials
✅ Upserted 15 transactions
✅ Upserted webhook endpoint
🎉 Seed completed successfully
📊 Totals => users: 6, modules: 8, completions: 12, credentials: 8, transactions: 15

(Note: Totals remain exactly the same between runs, proving the idempotent nature of the seeding script.)

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