fix: harden upstream feed fetching

[phatpham9]

Summary

• replace the Hacker News fetch path from hnrss.org/frontpage to Algolia's direct front-page API
• apply the configured outbound FEEDREADER_USER_AGENT to upstream requests
• add a bounded shared retry path for upstream GET fetches used by all feed sources
• add parser and transport/retry tests for the new behavior

Root cause

The Hacker News source depended on https://hnrss.org/frontpage, an extra third-party hop that intermittently stalled before response headers arrived, surfacing context deadline exceeded (Client.Timeout exceeded while awaiting headers).

The repo also documented FEEDREADER_USER_AGENT, but the HTTP client never actually applied it.

Details

• internal/sources/hackernews.go
  • switched HN ingestion from RSS-on-hnrss.org to direct Algolia front-page JSON
  • preserved story identity, comments URL, score, comments count, and published time mapping
• internal/service/service.go
  • wrapped the shared client transport to inject the configured User-Agent unless a request already sets one
• internal/sources/util.go
  • added bounded retry logic for upstream GETs
  • retries transport errors and retryable status codes (408, 425, 429, 500, 502, 503, 504)
• internal/sources/{github,huggingface,alphaxiv,hackernews}.go
  • moved source fetches onto the shared retry path
• tests
  • added HN parser coverage
  • added User-Agent transport coverage
  • added retry helper coverage

Verification

docker run --rm -v "$PWD":/src -w /src golang:1.24 /bin/sh -lc '/usr/local/go/bin/gofmt -w internal/service/service.go internal/service/http_test.go internal/sources/util.go internal/sources/hackernews.go internal/sources/hackernews_test.go internal/sources/http_test.go internal/sources/github.go internal/sources/huggingface.go internal/sources/alphaxiv.go && /usr/local/go/bin/go test ./...'