RM-335: Add channel flags and obfuscated channel detection

[biast12]

RM-335

Description

Discord is starting to obfuscate channels the bot can't view: from 2026-11-16 the gateway sends them with flag 1 << 17 and the name ___hidden___, and the HTTP API leaves them out. This adds ChannelFlag (including ChannelFlagObfuscated), a Flags field on Channel, CachedChannel and PartialChannel (carried through their conversions), and IsObfuscated() helpers.

For:
TicketsBot-cloud/worker#155
TicketsBot-cloud/dashboard#155

Type of Change

• Bug fix
• New feature
• Breaking change

Testing

• Checked that a gateway payload with flags: 131072 decodes, survives the cache round trip, and reports IsObfuscated()

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