feat: add gzip toggle and cache mode (normal / no-cache / bust)

- Cache-mode selector: Normal, No-cache headers, Cache-bust URL+headers
- Cache-bust appends random ?_cb= per iteration to bypass CDN/proxy
- gzip toggle: on = k6 default (Accept-Encoding: gzip/br), off = identity
- Both options stored in DB with non-destructive migration
- History items show cache-mode and gzip pills
- Schema migration handles existing DBs gracefully

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

frontend/index.html

@@ -58,6 +58,27 @@
             <input type="number" id="rpsLimit" min="0" value="0" />
           </div>
 
+          <div class="form-group">
+            <label for="cacheMode">Cache Mode</label>
+            <select id="cacheMode">
+              <option value="normal">Normal (allow cache)</option>
+              <option value="no-cache">No-cache headers (revalidate)</option>
+              <option value="bust">Cache-bust URL + headers (full bypass)</option>
+            </select>
+          </div>
+
+          <div class="form-group">
+            <label>Encoding</label>
+            <div class="toggle-row">
+              <label class="toggle">
+                <input type="checkbox" id="gzip" checked />
+                <span class="toggle-track"></span>
+                <span class="toggle-label">Accept gzip / br</span>
+              </label>
+            </div>
+            <div class="hint" id="gzip-hint">k6 default — server may compress response</div>
+          </div>
+
           <div class="form-group full" id="body-group" style="display:none">
             <label for="requestBody">Request Body</label>
             <textarea id="requestBody" rows="4" placeholder='{"key": "value"}'></textarea>
@@ -65,7 +86,7 @@
 
           <div class="form-group full">
             <label for="headers">
-              Custom Headers <span class="hint">(JSON, optional)</span>
+              Custom Headers <span class="hint">(JSON, optional — merged with above options)</span>
             </label>
             <input type="text" id="headers" placeholder='{"Authorization": "Bearer token"}' />
           </div>