bug: Fail fast when `mcp serve` starts without `seerr.server` configured

[electather]

Summary

mcp serve can start successfully even when seerr.server is not configured.

This happens because cmd/root.go intentionally skips the global config validation for the mcp command tree. The result is that the server starts and only fails later when the first tool call attempts to resolve the upstream Seerr base URL.

Affected code

• cmd/root.go
• cmd/mcp/serve.go
• cmd/apiutil/client.go

Current behaviour

• mcp serve starts
• first tool invocation fails at runtime because the upstream Seerr server URL is missing or malformed

Expected behaviour

For normal MCP use, startup should fail fast with a clear error if seerr.server is not configured.

Suggested fix

Add explicit validation in runServe() for the required Seerr configuration, rather than relying on the root command pre-run hook.

This can still allow future exceptions if there is ever a true no-upstream mode, but today the MCP tools all depend on a configured Seerr server.