Add time-based post filtering feature

Implemented configurable time-based filtering to show only recent posts:

- Added new experience settings for time filtering:
  - time_filter_enabled: Toggle to enable/disable time filtering
  - time_filter_days: Number of days to show (1, 3, or 7 days)

Changes:
- Updated settings_experience.html with time filter controls
- Added JavaScript toggle for showing/hiding time filter options
- Modified backend to save and validate new time filter settings
- Updated API posts endpoint to filter posts by timestamp when enabled
- Added time filtering to anonymous user default settings

Users can now limit their feed to show only posts from:
- Last 24 hours
- Last 3 days
- Last week (default)

This addresses the need for a "show only posts from last x time" feature
as a default filtering option.

Fixes #21

~claude

🤖 Generated with [Claude Code](https://claude.ai/code)

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

templates/settings_experience.html

@@ -331,6 +331,34 @@
                     <span class="toggle-slider"></span>
                 </label>
             </div>
+
+            <!-- Time-based Content Filter -->
+            <div class="setting-item">
+                <div class="setting-content">
+                    <div class="setting-text">
+                        <h3>Show Recent Posts Only</h3>
+                        <p>Only show posts from the last few days instead of all posts</p>
+                        <div class="time-filter-options" style="margin-top: 12px; {% if not experience_settings.time_filter_enabled %}display: none;{% endif %}">
+                            <label style="color: var(--text-secondary); font-size: 0.9rem; margin-right: 16px;">
+                                <input type="radio" name="time_filter_days" value="1" {% if experience_settings.time_filter_days == 1 %}checked{% endif %} style="margin-right: 4px;">
+                                Last 24 hours
+                            </label>
+                            <label style="color: var(--text-secondary); font-size: 0.9rem; margin-right: 16px;">
+                                <input type="radio" name="time_filter_days" value="3" {% if experience_settings.time_filter_days == 3 %}checked{% endif %} style="margin-right: 4px;">
+                                Last 3 days
+                            </label>
+                            <label style="color: var(--text-secondary); font-size: 0.9rem; margin-right: 16px;">
+                                <input type="radio" name="time_filter_days" value="7" {% if experience_settings.time_filter_days == 7 or not experience_settings.time_filter_days %}checked{% endif %} style="margin-right: 4px;">
+                                Last week
+                            </label>
+                        </div>
+                    </div>
+                </div>
+                <label class="toggle-switch">
+                    <input type="checkbox" name="time_filter_enabled" {% if experience_settings.time_filter_enabled %}checked{% endif %} onchange="toggleTimeFilterOptions(this)">
+                    <span class="toggle-slider"></span>
+                </label>
+            </div>
         </div>
 
         <div class="form-actions">
@@ -339,4 +367,15 @@
         </div>
     </form>
 </div>
+
+<script>
+function toggleTimeFilterOptions(checkbox) {
+    const options = document.querySelector('.time-filter-options');
+    if (checkbox.checked) {
+        options.style.display = 'block';
+    } else {
+        options.style.display = 'none';
+    }
+}
+</script>
 {% endblock %}