feat(cache): implement integrity checks for cached events and waveforms

- Added `waveform_key` and `event_timestamp` columns to `CachedEvent` and `CachedWaveform` for integrity verification.
- Implemented logic to flush the cache when a mismatch in (waveform_key, event_timestamp) is detected during event and waveform updates.
- Enhanced `set_events` and `set_waveform` methods to check for mismatches and trigger cache eviction as necessary.
- Introduced a new `LiveCache` class to manage in-memory caching of live device data, separating it from the server logic for better testability.
- Added tests to verify the correctness of cache invalidation logic, particularly for post-erase key reuse scenarios.
- Updated web application to include a "Force refresh" toggle, allowing users to bypass the cache and re-fetch data from the device.

sfm/sfm_webapp.html

@@ -609,6 +609,36 @@
     .section-btn:hover  { color: var(--text); }
     .section-btn.active { background: var(--blue); color: #fff; }
 
+    /* ── Force-refresh toggle ── */
+    .force-toggle {
+      display: flex;
+      align-items: center;
+      gap: 6px;
+      padding: 4px 10px;
+      border: 1px solid var(--border);
+      border-radius: 6px;
+      background: var(--bg);
+      cursor: pointer;
+      font-size: 11px;
+      font-weight: 600;
+      color: var(--text-dim);
+      user-select: none;
+      white-space: nowrap;
+      transition: background 0.12s, color 0.12s, border-color 0.12s;
+    }
+    .force-toggle input { margin: 0; cursor: pointer; }
+    .force-toggle:hover { color: var(--text); }
+    .force-toggle.active {
+      background: rgba(248, 81, 73, 0.18);
+      border-color: #f85149;
+      color: #ff7b72;
+    }
+    .force-toggle .ft-dot {
+      width: 6px; height: 6px; border-radius: 50%;
+      background: var(--text-mute);
+    }
+    .force-toggle.active .ft-dot { background: #f85149; box-shadow: 0 0 6px #f85149; }
+
     /* ── Section containers ── */
     #section-live, #section-db {
       display: flex;
@@ -654,6 +684,13 @@
     <button class="section-btn active" onclick="switchSection('live')">Live Device</button>
     <button class="section-btn"        onclick="switchSection('db')">Database</button>
   </div>
+  <div class="hdr-sep"></div>
+  <label class="force-toggle" id="force-toggle"
+         title="Bypass server cache and dedup. Forces a fresh download from the device on every live request — useful when the device has been erased and the cache is showing stale events.">
+    <input type="checkbox" id="force-cb" onchange="onForceToggle()">
+    <span class="ft-dot"></span>
+    <span>Force refresh</span>
+  </label>
 </header>
 
 <!-- ════════════════════════════════════════════════════════════════
@@ -1214,8 +1251,21 @@ function setCfgStatus(msg, cls = '') {
   el.className = cls;
 }
 
+// "Force refresh" override — when enabled, every live-device request is
+// sent with ?force=true so the server bypasses its in-memory + persistent
+// caches and re-reads from the device.  Manual escape hatch for cases where
+// the cache has gone stale (e.g. post-erase key reuse — see ach_server.py
+// and sfm/cache.py for the eviction logic).
+let forceRefresh = false;
+
+function onForceToggle() {
+  forceRefresh = document.getElementById('force-cb').checked;
+  document.getElementById('force-toggle').classList.toggle('active', forceRefresh);
+}
+
 function deviceParams() {
-  return `host=${encodeURIComponent(devHost())}&tcp_port=${devPort()}`;
+  const base = `host=${encodeURIComponent(devHost())}&tcp_port=${devPort()}`;
+  return forceRefresh ? `${base}&force=true` : base;
 }
 
 // ── Section switching ─────────────────────────────────────────────────────────