Add community Discord XP tracking

[JoaquinBN]

Summary

• Adds per-contribution Discord XP tracking for accepted community contributions.
• Adds steward Discord XP page with permission-scoped rows, command copy, distribution flagging, and unset support.
• Adds read-only Django admin visibility for XP state and audit events.
• Refreshes Discord usernames by stable Discord user ID before copying XP commands.

Notes

• XP visibility and actions are scoped to contribution types where the steward has accept permission.
• Zero-point rows remain hidden unless XP was already distributed, so reduced contributions can still be reviewed and unset.