refactor: remove test.db, use only posterg.db for all environments

- Simplified Database.php determineDatabasePath to always use posterg.db
- Removed test.db auto-detection based on php_sapi_name
- Removed test.db targets from justfile (migrate-test removed)
- Removed CreateTestDatabase.php fixture script
- Updated migrate.sh to only init posterg.db
- Updated setup-dev.sh to init posterg.db
- Updated run-tests.php (removed DB_ENV=test env var)
- Updated deploy-db to use posterg.db
- Removed test.db file

refactor: remove empty fixtures directory

app/tests/run-tests.php

@@ -5,10 +5,6 @@
  * Runs all tests in the tests/ directory
  */
 
-// Tests always run against the test database; require an explicit opt-in so
-// that a stray test.db on disk never silently redirects a production session.
-putenv('DB_ENV=test');
-
 echo "╔════════════════════════════════════════════╗\n";
 echo "║         Post-ERG Test Suite                ║\n";
 echo "╚════════════════════════════════════════════╝\n\n";