Now that KanzlAI is on the youpc.org Supabase instance, the separate
YouPCDatabaseURL connection is unnecessary. The main database connection
can query mlex.* tables directly since they're on the same Postgres.
- Remove YouPCDatabaseURL from config
- Remove separate sqlx.Connect block in main.go
- Pass main database handle as youpcDB parameter to router
- Update CLAUDE.md: mgmt schema in youpc.org (was kanzlai in flexsiebels)
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