fix: remove DB_ENV auto-detection; require explicit DB_ENV=test for tests

src/config.php: remove the file-existence fallback that silently redirected
all requests to test.db whenever that file was present on disk. getDatabasePath()
now always returns the production DB unless DB_ENV=test is explicitly set.

tests/run-tests.php: putenv('DB_ENV=test') at the top so the suite always
targets test.db regardless of what is set in the shell environment.

tests/Unit/DatabaseTest.php, tests/Integration/SearchTest.php,
tests/Security/SecurityTest.php: same putenv() guard added to each file so
they work correctly when run standalone (e.g. just test-unit).

justfile: all test and DB-development recipes now prefix DB_ENV=test to their
php/sqlite3 commands, making the intent explicit in the recipe itself.

Fixes: a developer who ran the test suite and kept test.db on disk would
silently hit test data when browsing the local site with no DB_ENV set.

tests/Unit/DatabaseTest.php

@@ -4,6 +4,9 @@
  * Tests basic database connectivity and query functionality
  */
 
+// Must be set before Database.php is required so getDatabasePath() picks it up.
+putenv('DB_ENV=test');
+
 require_once __DIR__ . '/../../src/Database.php';
 
 echo "Database Connection Test\n";