fix: skip db boot init in test env to prevent parallel worker lock

Vitest runs test files in parallel workers. Each worker imports server/db.js,
which triggered module-level init(DEFAULT_PATH) unconditionally. Two workers
racing to open the same SQLite file caused "database is locked", followed
by process.exit(1) killing the worker — surfacing as:

  Error: process.exit unexpectedly called with "1"

Fix: guard the boot init block behind NODE_ENV !== 'test'. Vitest sets
NODE_ENV=test automatically. Each worker's beforeEach(() => _resetForTest())
initialises its own :memory: database, so no file coordination is needed.

process.exit(1) is also guarded by the same condition — it must never
fire inside a test runner process.

TDD: two regression tests added to tests/db.test.js documenting the
expected boot behaviour and proving the module loads cleanly in parallel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/db.js

@@ -133,5 +133,18 @@ export function _resetForTest() {
 }
 
 // ── Boot ──────────────────────────────────────────────────────────────────────
+// Skipped in test environment — parallel Vitest workers would race to open
+// the same file, causing "database is locked". _resetForTest() in beforeEach
+// handles initialisation for every test worker using :memory: instead.
 
-init(process.env.DB_PATH ?? DEFAULT_PATH);
+if (process.env.NODE_ENV !== 'test') {
+  const DB_PATH = process.env.DB_PATH ?? DEFAULT_PATH;
+  try {
+    init(DB_PATH);
+  } catch (e) {
+    console.error('[catalyst] fatal: could not open database at', DB_PATH);
+    console.error('[catalyst] ensure the data directory exists and is writable by the server process.');
+    console.error(e);
+    process.exit(1);
+  }
+}