refactor: use Chain type for FilBeam retriever config

[bajtos]

Replace FilecoinNetworkType with Chain from synapse-core in FilBeamRetriever, reading the retrieval domain from chain configuration instead of hardcoding network-to-domain mapping.

Refs:

• This is a follow-up for refactor: move FilBeam domain to chain configuration #570
• Targeting refactor: viem in payments #569

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Status	Name	Latest Commit	Updated (UTC)
❌ Deployment failed View logs	synapse-dev	4b9d592	Jan 27 2026, 02:51 PM

[bajtos]

@rvagg re: #570 (comment)

a better port number might be advisable because I think we include it in the default retrieval path

Would you like me to change the devnet FilBeam domain in this PR? What value do you suggest?

[bajtos]

Re-posting another relevant comment:

How do you feel about using filbeam.localhost:8080 for devnet? It won't work out of the box, but it's possible to configure your machine to make that hostname work, plus the hostname is IMO descriptive enough to make it clear what's going on when somebody tries FilBeam retrievals for devnet pieces.

[bajtos]

There is one more caveat that I realised just now: We are always using HTTPS protocol for FilBeam retrievals. That's fine for mainnet and calibration testnet, but probably too difficult to set up for local devnet 🤔