Add file cleanup callback for uploaded files (issues #264, #270)

[etr]

Summary

This PR adds a server-level callback that allows users to customize file cleanup behavior for uploaded files, addressing issues #264 and #270.

Problem: Previously, all uploaded files were automatically deleted when the request completed, making it impossible to keep or move files to permanent storage.

Solution: Add a file_cleanup_callback option that is invoked for each uploaded file when the request completes. The callback can:

• Return true to delete the file (default behavior)
• Return false to keep the file (e.g., after moving it to permanent storage)

Usage Example

webserver ws = create_webserver(8080)
    .file_upload_target(FILE_UPLOAD_DISK_ONLY)
    .file_upload_dir("/tmp/uploads")
    .file_cleanup_callback([](const std::string& key,
                              const std::string& filename,
                              const http::file_info& info) {
        // Move file to permanent storage
        std::string dest = "/var/uploads/" + filename;
        std::rename(info.get_file_system_file_name().c_str(), dest.c_str());
        return false;  // Don't delete - we moved it
    });

Changes

• src/httpserver/create_webserver.hpp: Add file_cleanup_callback_ptr typedef and builder method
• src/httpserver/webserver.hpp: Add callback storage member
• src/webserver.cpp: Initialize callback and pass to http_request
• src/httpserver/http_request.hpp: Add callback member and setter
• src/http_request.cpp: Modify destructor to invoke callback per file
• test/integ/file_upload.cpp: Add 5 test cases covering callback behavior
• examples/file_upload_with_callback.cpp: New example demonstrating the feature
• examples/Makefile.am: Add new example to build
• README.md: Documentation and usage example

Test plan

• All existing tests pass (make check)
• New test: callback returns true → file deleted
• New test: callback returns false → file kept
• New test: selective cleanup (keep some, delete others)
• New test: callback throws → file deleted (safe default)
• New test: no callback → backward compatible deletion

Closes #264
Closes #270