RM-220: Fix guild purge failing on foreign keys and dropped tables

[biast12]

Description

PurgeGuildData has a hardcoded table list that nothing keeps in sync with the schema. It was correct when written (ea0f319, 2026-01-06) and has drifted since, so guild purges fail and the guild is retried every 6 hours forever.

Two independent faults:

1. DELETE FROM tickets raises 23503. The 2026.08 release (53077b5) added ticket_message_counts with FOREIGN KEY(guild_id, ticket_id) REFERENCES tickets(...) and no ON DELETE action, and never added it to the purge list. webhooks has the same FK shape and is in the list, but sits 41 entries after tickets — dormant only because webhook rows exist solely for open tickets.
2. Five names in the list have no CREATE TABLE any more. 53077b5 deleted the archive_channel, naming_scheme, ticket_limit, ticket_permissions and welcome_messages wrappers but left them in guildpurge.go. A DELETE against a missing relation raises 42P01 and aborts the whole transaction.

For: TicketsBot-cloud/cleanupdaemon#4

Type of Change

• Bug fix
• New feature
• Breaking change

Checklist

• My code follows the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[BenHall-1]

Good to bump common when you're ready @biast12

[biast12]

Bumped!!!