feat(import): preserve supabase user ids behind --source-preserve-ids

[LeadcodeDev]

Closes #46.

--source-preserve-ids creates each user with the id it already has in Supabase, so the sub a relying party stores keeps resolving after the migration. Off by default, Supabase-only, and refused on the other sources the same way --source-passwords is.

The id stays a String end to end rather than becoming a Uuid. Nothing in the CLI parses one today, the server validates the format, and the only comparison needed is against the id it echoes back — done case-insensitively, since neither side promises a case.

A server built before ferriskey/ferriskey#1495 ignores the field and mints its own id. The import compares what came back against what it asked for and returns an error on the first mismatch instead of warning and carrying on: a half-migrated directory where some accounts kept their subject and others did not is worse than one that stopped. A 409 carrying user_id_already_exists is told apart from a plain duplicate username for the same reason, since the existing path would have logged it as "already exists, reusing it" and skipped an account that was never written.

Not exercised against a live Supabase project or a live FerrisKey server. The unit tests cover the mapping, the request body and the two failure paths; the end-to-end proof that a supplied id becomes the token subject lives in ferriskey's own supplied_user_id_test.

cargo test --workspace is 163 passing, cargo clippy --workspace --all-targets --all-features -- -D warnings clean.

[coderabbitai]

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