fix: add User-Agent to MetadataFetcher HTTP client to prevent CDN 403s

[pthmas]

Problem

MetadataFetcher builds its reqwest client with no User-Agent header (metadata.rs:47-50). CDNs serving NFT metadata (IPFS gateways, OpenSea, custom CDNs) classify requests with no User-Agent as bot scrapers and return 403 Forbidden. This means Atlas never fetches the metadata JSON, image_url stays null in the database, and images are blank for every token served from those CDNs.

Reported error in indexer logs:

DEBUG atlas_server::indexer::metadata: failed to fetch token metadata contract=0x... token_id=13240461 error="HTTP 403 Forbidden"

Fix

Add .user_agent("atlas-server/0.1.0") to the reqwest::Client::builder() call in MetadataFetcher::new (metadata.rs:47):

let client = reqwest::Client::builder()
    .timeout(Duration::from_secs(30))
    .user_agent("atlas-server/0.1.0")
    .build()
    .expect("Failed to create HTTP client");

No new config fields needed — hardcoding the string is sufficient to pass CDN bot filters. A contact URL suffix (+https://...) is a courtesy convention but not required.