chore: use isolated test database instead of dev DB

[Flo0806]

Problem

All unit tests currently use getDb() which connects to the development database (data/dm-hero.db). While tests create their own campaigns and clean up after themselves, this is risky - a bug in cleanup could corrupt dev data.

Solution

Create a getTestDb() utility that:

• Creates a separate in-memory or temp-file SQLite database
• Runs all migrations on it
• Returns the isolated DB instance

Then migrate all existing test files to use getTestDb() instead of getDb().

Files affected

All files in packages/app/test/unit/ that import getDb from server/utils/db.