feat(cache): add L2 disk cache for original fetched files (HTTP/S3)

[nakamuraos]

Summary

• Adds OriginCache that stores raw fetched bytes (before transforms) to disk under <cache_dir>/origin/,
  keyed by SHA-256 of the source URL
• On cache miss for a transformed request, checks origin cache first - if the source URL was previously
  fetched, skips the origin fetch and runs transforms on cached bytes
• Supports per-entry TTL: HTTP streaming path extracts Cache-Control: max-age / Expires headers from
  origin response; buffered path falls back to PP_CACHE_DISK_TTL_SECS
• Applies to HTTP and S3 sources only; local and alias sources are excluded

Test Plan

• cargo test - 396 tests pass
• Request same URL with different transform params - second request should skip origin fetch (check logs
  for "origin cache hit")
• Verify <cache_dir>/origin/ directory is populated after first request
• Verify TTL-expired entries are cleaned up on next cleanup cycle