Add 36 new E2E tests across 5 test files

New E2E test files:
- e2e/health.spec.ts: 3 tests for health/observability endpoints
- e2e/history.spec.ts: 7 tests for history page
- e2e/plan.spec.ts: 7 tests for exercise plan page
- e2e/decision-engine.spec.ts: 8 tests for decision display and overrides
- e2e/cycle.spec.ts: 11 tests for cycle tracking, settings, and period logging

Total E2E tests: 100 (up from 64)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

e2e/health.spec.ts

@@ -0,0 +1,49 @@
+// ABOUTME: E2E tests for health and observability endpoints.
+// ABOUTME: Tests health check endpoint response and performance.
+import { expect, test } from "@playwright/test";
+
+test.describe("health and observability", () => {
+  test.describe("health endpoint", () => {
+    test("health endpoint returns 200 when healthy", async ({ request }) => {
+      const response = await request.get("/api/health");
+
+      expect(response.status()).toBe(200);
+
+      const body = await response.json();
+      expect(body.status).toBe("ok");
+      expect(body).toHaveProperty("timestamp");
+      expect(body).toHaveProperty("version");
+    });
+
+    test("health endpoint responds quickly", async ({ request }) => {
+      const startTime = Date.now();
+      const response = await request.get("/api/health");
+      const endTime = Date.now();
+
+      expect(response.status()).toBe(200);
+      // E2E includes network latency; allow 500ms for full round-trip
+      // (the handler itself executes in <100ms per spec)
+      expect(endTime - startTime).toBeLessThan(500);
+    });
+  });
+
+  test.describe("metrics endpoint", () => {
+    test("metrics endpoint is accessible and returns Prometheus format", async ({
+      request,
+    }) => {
+      const response = await request.get("/api/metrics");
+
+      expect(response.status()).toBe(200);
+
+      const contentType = response.headers()["content-type"];
+      expect(contentType).toContain("text/plain");
+
+      const body = await response.text();
+      // Prometheus format should contain HELP and TYPE comments
+      expect(body).toMatch(/^# HELP/m);
+      expect(body).toMatch(/^# TYPE/m);
+      // Should contain our custom metrics
+      expect(body).toContain("phaseflow_");
+    });
+  });
+});