fix: make seed_db idempotent and require explicit --reset to drop tables (#1856)

[ionfwsrijan]

Problem

src/seed_db.py starts with an unconditional db.drop_all(), wiping every table — users, saved progress, game progress and admin flags — every time the documented seeding script is run on an existing database. It also inserts without the project validation that the data loader relies on.

Fix

• src/seed_db.py: replace db.drop_all() with an idempotent upsert — db.create_all() then update-or-insert each project by id (matching the app.py auto-seed behavior) — so running the seed script no longer destroys user data.
• The destructive path is now guarded behind an explicit --reset flag with a confirmation prompt.
• The dataset is validated with validate_projects() before any writes, and the whole upsert is committed in one transaction.
• tests/test_basic.py: add test_seed_database_is_idempotent_and_preserves_users verifying a user survives re-seeding and the project count stays correct.

Files changed

• src/seed_db.py
• tests/test_basic.py

Testing

• py -m pytest tests/ -q → 522 passed, 19 pre-existing unrelated failures (identical to baseline), 3 skipped.
• New test confirms seeding preserves existing user rows.

Closes #1856

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