fix(build): make CURL optional so dflash_server links without libcurl

[dusterbloom]

Re-carved from #274 as a standalone change off current main.

Gates all libcurl usage in http_server.cpp behind DFLASH_HAS_CURL; CMake builds dflash_server unconditionally and only defines the macro + links CURL::libcurl when find_package(CURL) succeeds. Without libcurl the passthrough proxy compiles out and the server still builds.

Re-carved (not cherry-picked): #294's passthrough-proxy added two curl_forward call sites (http_server.cpp:1888, :1909) beyond the original commit; both are now gated.

Verified on a box without libcurl-dev: cmake configure succeeds and http_server.cpp compiles clean with DFLASH_HAS_CURL undefined. 2 files, +34/-24.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}