Drop the single-database split and re-baseline the control migrations

Every deployment has been through the per-blog split, so the code that
performed it (split.go, control migrations 00002–00007) is dead weight, and a
fresh install replaying six migrations only to drop the tables again was
silly. The control chain is now a single 00001_init.sql with the final users
and blogs shape, matching the blog chain.

Goose ignores versions recorded in the database that no longer exist in the
source, but refuses a future migration numbered below the database's highest
version. Databases that went through the split therefore need
`DELETE FROM goose_db_version WHERE version_id > 1` once; README and
AGENTS.md say so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

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