Seed data export/import composability

[toby]

We currently import the seed.json when the MCP_REGISTRY_SEED_FILE_PATH environment variable is set and MCP_REGISTRY_SEED_IMPORT=true.

The registry should expose this file at a publicly accessible endpoint and allow for importing it from a http based location. That way you can start a new instance of the registry, point it at an existing registry, and have the new registry import all of the MCP servers from the original registry. The seed.json endpoint should expose the data in the exact same format as needed for the import. We can continue to allow the seed file to be imported from the filesystem.

We can also use this as an opportunity to clean up the UX around importing a bit.

• Create a /v0/seed.json endpoint that returns a full list of servers in the seed format
• Remove the MCP_REGISTRY_SEED_IMPORT env var and instead import if the MCP_REGISTRY_SEED_FILE_PATH is set
• Add a --seed-file-path flag that does the same thing as the MCP_REGISTRY_SEED_FILE_PATH env var

[domdomegg]

I think this is superceded by #175, where we went with an approach of being able to seed from another registry endpoint. Reopen if I've misunderstood the underlying requirement/need here :)