feat(FN-4025): defer database integrity check and expose via health endpoin

Exposes database health status via the API by deferring integrity checks asynchronously, with docs and tests covering the core DB layer, store integration, and dashboard server health endpoint.

Fusion-Task-Id: FN-4025

packages/core/src/__tests__/db.test.ts

@@ -269,23 +269,52 @@ describe("Database", () => {
   });
 
   describe("startup integrity check", () => {
-    it("passes silently on a healthy database", () => {
-      // db was already init'd in beforeEach — no warning means pass
-      const result = db.prepare("PRAGMA integrity_check").get() as { integrity_check: string };
-      expect(result.integrity_check).toBe("ok");
-    });
+    it("schedules full integrity check after init instead of blocking startup", () => {
+      vi.useFakeTimers();
+      const integritySpy = vi.spyOn(Database.prototype, "integrityCheck");
 
-    it("init completes without throwing even on a fresh database", () => {
       const freshDir = makeTmpDir();
       const freshFusionDir = join(freshDir, ".fusion");
       const freshDb = new Database(freshFusionDir);
 
       try {
-        // init includes the integrity check — should not throw
         expect(() => freshDb.init()).not.toThrow();
+        expect(freshDb.integrityCheckPending).toBe(true);
+        expect(integritySpy).not.toHaveBeenCalled();
+
+        vi.advanceTimersByTime(3000);
+
+        expect(integritySpy).toHaveBeenCalledTimes(1);
+        expect(freshDb.integrityCheckPending).toBe(false);
+        expect(freshDb.integrityCheckLastRunAt).toBeTruthy();
       } finally {
         freshDb.close();
         rmSync(freshDir, { recursive: true, force: true });
+        integritySpy.mockRestore();
+        vi.useRealTimers();
+      }
+    });
+
+    it("does not schedule duplicate background integrity checks across repeated init calls", () => {
+      vi.useFakeTimers();
+      const integritySpy = vi.spyOn(Database.prototype, "integrityCheck");
+      const freshDir = makeTmpDir();
+      const freshFusionDir = join(freshDir, ".fusion");
+      const freshDb = new Database(freshFusionDir);
+
+      try {
+        freshDb.init();
+        expect(freshDb.integrityCheckPending).toBe(true);
+
+        freshDb.init();
+        vi.advanceTimersByTime(3000);
+
+        expect(integritySpy).toHaveBeenCalledTimes(1);
+      } finally {
+        freshDb.close();
+        rmSync(freshDir, { recursive: true, force: true });
+        integritySpy.mockRestore();
+        vi.useRealTimers();
       }
     });
   });
@@ -656,14 +685,17 @@ describe("Database", () => {
       const diskDb = new Database(fusionDir);
       diskDb.init();
       expect(diskDb.corruptionDetected).toBe(false);
+      expect(diskDb.integrityCheckPending).toBe(true);
       diskDb.close();
     });
 
-    it("skips integrity check side effects for in-memory databases", () => {
+    it("skips background integrity check scheduling for in-memory databases", () => {
       const memDb = new Database(fusionDir, { inMemory: true });
       memDb.init();
       expect(memDb.integrityCheck()).toEqual({ ok: true });
       expect(memDb.corruptionDetected).toBe(false);
+      expect(memDb.integrityCheckPending).toBe(false);
+      expect(memDb.integrityCheckLastRunAt).toBeNull();
       memDb.close();
     });
   });