feat: invalidate cache on web uploads and opds downloads and add Clear Cache action (#393)

## Summary

When uploading or downloading an updated ebook from SD/WebUI/OPDS with
same the filename the `.crosspoint` cache is not cleared. This can lead
to issues with the Table of Contents and hangs when switching between
chapters.

I encountered this issue in two places:
- When I need to do further ePub cleaning using Calibre after I load an
ePub and find that some of its formatting should be cleaned up. When I
reprocess the same book and want to place it back in the same location I
need a way to invalidate the cache.
- When syncing RSS feed generated epubs. I generate news ePubs with
filenames like `news-outlet.epub` and so every day when I fetch new news
the crosspoint cache needs to be cleared to load that file.

This change offers the following features:
- On web uploads, if the file already exists, the cache for that file is
cleared
- On OPDS downloads, if the file already exists, the cache for that file
is cleared
- There's now an action for `Clear Cache` in the Settings page which can
clear the cache for all books


Addresses
https://github.com/crosspoint-reader/crosspoint-reader/issues/281

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? PARTIALLY

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/network/CrossPointWebServer.cpp

@@ -1,6 +1,7 @@
 #include "CrossPointWebServer.h"
 
 #include <ArduinoJson.h>
+#include <Epub.h>
 #include <FsHelpers.h>
 #include <SDCardManager.h>
 #include <WiFi.h>
@@ -10,6 +11,7 @@
 
 #include "html/FilesPageHtml.generated.h"
 #include "html/HomePageHtml.generated.h"
+#include "util/StringUtils.h"
 
 namespace {
 // Folders/files to hide from the web interface file browser
@@ -28,6 +30,15 @@ size_t wsUploadSize = 0;
 size_t wsUploadReceived = 0;
 unsigned long wsUploadStartTime = 0;
 bool wsUploadInProgress = false;
+
+// Helper function to clear epub cache after upload
+void clearEpubCacheIfNeeded(const String& filePath) {
+  // Only clear cache for .epub files
+  if (StringUtils::checkFileExtension(filePath, ".epub")) {
+    Epub(filePath.c_str(), "/.crosspoint").clearCache();
+    Serial.printf("[%lu] [WEB] Cleared epub cache for: %s\n", millis(), filePath.c_str());
+  }
+}
 }  // namespace
 
 // File listing page template - now using generated headers:
@@ -500,6 +511,12 @@ void CrossPointWebServer::handleUpload() const {
                       uploadFileName.c_str(), uploadSize, elapsed, avgKbps);
         Serial.printf("[%lu] [WEB] [UPLOAD] Diagnostics: %d writes, total write time: %lu ms (%.1f%%)\n", millis(),
                       writeCount, totalWriteTime, writePercent);
+
+        // Clear epub cache to prevent stale metadata issues when overwriting files
+        String filePath = uploadPath;
+        if (!filePath.endsWith("/")) filePath += "/";
+        filePath += uploadFileName;
+        clearEpubCacheIfNeeded(filePath);
       }
     }
   } else if (upload.status == UPLOAD_FILE_ABORTED) {
@@ -787,6 +804,12 @@ void CrossPointWebServer::onWebSocketEvent(uint8_t num, WStype_t type, uint8_t*
         Serial.printf("[%lu] [WS] Upload complete: %s (%d bytes in %lu ms, %.1f KB/s)\n", millis(),
                       wsUploadFileName.c_str(), wsUploadSize, elapsed, kbps);
 
+        // Clear epub cache to prevent stale metadata issues when overwriting files
+        String filePath = wsUploadPath;
+        if (!filePath.endsWith("/")) filePath += "/";
+        filePath += wsUploadFileName;
+        clearEpubCacheIfNeeded(filePath);
+
         wsServer->sendTXT(num, "DONE");
         lastProgressSent = 0;
       }