gateway(legacy): port /get fetch pool to gateway.DefaultGateways; bump v0.18.1

[noslav]

Summary

Stacks on top of #73. #73 swapped the new-path gateway pool from w3s.link to ipfs.filebase.io in internal/gateway/handler.go, but the deprecated /get endpoint kept its own hardcoded gateway list in internal/pinner/ipfs-node/legacy.go:

IPFS_HTTP_GATEWAYS = []string{
    "https://w3s.link/ipfs/%s",
    "https://dweb.link/ipfs/%s",
    "https://ipfs.io/ipfs/%s",
}

So consumers still on /get (today: refiner, until very recently bsp-agent) were silently routed through w3s.link and saw it in the logs:

ewm-das  WARN  Deprecated endpoint accessed: /get
ewm-das  DEBUG ipfs-node/legacy.go:76  trying out https://w3s.link/ipfs/bafy...

This PR derives IPFS_HTTP_GATEWAYS from gateway.DefaultGateways at package init, so the legacy fetcher uses the same single source of truth as the /api/v1/get handler. The list now reads (post-#73):

https://ipfs.filebase.io/ipfs/%s
https://trustless-gateway.link/ipfs/%s
https://dweb.link/ipfs/%s
https://ipfs.io/ipfs/%s

Design notes

• Source of truth. gateway.DefaultGateways is now the single place to update the pool. Future tweaks flow through both endpoint families automatically and the legacy path can never drift back to a retired gateway.
• Format suffix. The legacy fetcher uses fmt.Sprintf to inject the CID, so each base URL is suffixed with /ipfs/%s at init (strings.TrimRight(g, "/") + "/ipfs/%s"). The new path uses url.Parse + path.Join and doesn't need the suffix.
• DEDICATED_GATEWAY intentionally not honoured here. The legacy endpoint is deprecated. Consumers should migrate to /api/v1/get to pick up dedicated-gateway support and the new path's parallel-fetch worker pool. Keeping the legacy path "as good as new" would weaken the migration incentive.
• No semantic change to the legacy fetcher. It still returns raw IPFS bytes (vs the new path's DAS-decode + reconstruct). Only the gateway list changes.

Version bump

common/constants.go is also bumped: Version = "v0.18.1" (was "v0.1.0", a stale fallback). The Makefile injects git describe --tags via ldflags at build-time so this constant is only used when the binary is built without the Makefile path — but the fallback should still report a sensible post-#73 version.

Test plan

• Merge Swap w3s.link → ipfs.filebase.io; add optional DEDICATED_GATEWAY #73 first (this PR's base).
• Rebuild ewm-das:latest from this branch.
• Re-run covalenthq/refiner PR #292 CI against the rebuilt image with /get still mapped — confirm no w3s.link line in the ewm-das logs.
• Hit /get?cid=<known-good-cid> against the rebuilt image directly — confirm it routes through ipfs.filebase.io first and returns the same bytes the old gateway list did.

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