fix(discord): de-dupe allowed_mentions roles/users to avoid silent 400

[poterpan]

Problem

When two plans share the same Discord role id (e.g. Claude Standard and Claude Premium both mapped to a single @Claude role), the billing-opened notice silently failed to send.

Root cause

sendBillingOpened builds one lines entry per active plan, then maps each line's role_id into allowed_mentions.roles without de-duping:

const roles = lines.map((l) => l.role_id).filter((r): r is string => !!r);
// two plans, one role → ["claude", "claude"]

Discord treats allowed_mentions.roles / .users as unique sets and returns HTTP 400 Invalid Form Body (code 50035) on duplicate snowflakes. createChannelMessage maps any non-2xx response to null (if (!res.ok) return null), so the failure is completely silent — the notice just never appears, no log, no throw.

Affects both the cron auto-open path and admin /發起繳費. The overdue path (sendOverdue) uses users grouped per-member so it's not hit today, but shares the same un-deduped shape.

Fix

De-dupe both arrays with Set:

• roles in sendBillingOpened (the actual bug)
• users in sendOverdue (defensive — kills the latent twin)

Per-plan display is unchanged: the content still renders one mention line per plan (the role is still only pinged once).

Tests

• New regression test: two plans sharing a role → allowed_mentions.roles is ["claude"] (not ["claude","claude"]), and the content still shows the mention on both plan lines.
• New regression test: duplicate member in overdue list → users de-duped.
• Full worker suite: 179 passing, typecheck clean.