Use direct HTTP API for IPFS instead of ipfshttpclient

Replace ipfshttpclient library with direct HTTP requests to IPFS API.
This fixes compatibility with newer Kubo versions (0.39.0+) which are
not supported by the outdated ipfshttpclient library.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

requirements.txt

@@ -6,6 +6,5 @@ uvicorn>=0.27.0
 python-multipart>=0.0.6
 PyYAML>=6.0
 asyncpg>=0.29.0
-ipfshttpclient>=0.7.0
 # Core artdag from GitHub
 git+https://github.com/gilesbradshaw/art-dag.git