refactor(domains): rename meadows to domains

Summary

• Rename the Meadow context, schemas, LiveViews, routes, CSS, Slack unfurls, webhooks, drop classification, forage associations, MCP arguments, seeds, and tests to Domain terminology so the code and UI use the new product taxonomy consistently.
• Add a reversible migration that renames the meadows tables, join tables, webhook tables, foreign keys, indexes, and constraints to domains while preserving historical migrations.
• Update README, self-hosting docs, in-repo guidance, feed URLs, and subscription parameters from /meadows and meadow_ids to /domains and domain_ids.
• Kept the change as a rename-focused refactor instead of adding compatibility aliases, because the request was to rename the concept across the project and the test suite covers the renamed runtime paths.

Testing

• mix format
• mix compile --warnings-as-errors
• MIX_ENV=test mix ecto.rollback -n 1 && MIX_ENV=test mix ecto.migrate
• mix test

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