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

@@ -133,7 +133,11 @@ 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.
 
+if (process.env.NODE_ENV !== 'test') {
   const DB_PATH = process.env.DB_PATH ?? DEFAULT_PATH;
   try {
     init(DB_PATH);
@@ -143,3 +147,4 @@ try {
     console.error(e);
     process.exit(1);
   }
+}

tests/db.test.js

@@ -165,3 +165,23 @@ describe('deleteInstance', () => {
     expect(getInstance(2)).not.toBeNull();
   });
 });
+
+// ── Test environment boot isolation ───────────────────────────────────────────
+
+describe('test environment boot isolation', () => {
+  it('vitest runs with NODE_ENV=test', () => {
+    // Vitest sets NODE_ENV=test automatically. This is the guard condition
+    // that prevents the boot init() from opening the real database file.
+    expect(process.env.NODE_ENV).toBe('test');
+  });
+
+  it('db module loads cleanly in parallel workers without locking the real db file', () => {
+    // Regression: the module-level init(DEFAULT_PATH) used to run unconditionally,
+    // causing "database is locked" when multiple test workers imported db.js at
+    // the same time. process.exit(1) then killed the worker mid-suite.
+    // Fix: boot init is skipped when NODE_ENV=test. _resetForTest() handles setup.
+    // Reaching this line proves the module loaded without calling process.exit.
+    expect(() => _resetForTest()).not.toThrow();
+    expect(getInstances()).toEqual([]);
+  });
+});