[Split Tracking] Add `split_gp_tracking` group configuration toggle

[joelhalen]

Overview

Part of #28 — Split GP Tracking feature.

Groups need an opt-in flag to enable split GP tracking. This is stored as a config key in the existing group_configurations table (same GroupConfiguration model already used for all per-group settings).

Work Required

Config key

• Key: split_gp_tracking
• Values: "1" = enabled, "0" or absent = disabled (default)
• No schema change needed — group_configurations is a generic key/value table.

New Discord admin command: /toggle-split-tracking

File: commands/admin.py — ClanCommands extension

• Requires ADMINISTRATOR permission (matches all existing group commands).
• Resolves group_id from ctx.guild_id.
• Reads current value of the split_gp_tracking config key for that group.
• Toggles it (0 → 1 or 1 → 0), upserting a GroupConfiguration row.
• Responds with an ephemeral message: "Split GP tracking is now enabled / disabled for {group_name}."

Follow the pattern established by /reset-group-points (line 301) and /force-group-sync (line 357) for the DB lookup + config upsert.

Notes

• When a group disables this flag after it was previously enabled, no historical drop_splits rows need to be cleaned up — they simply won't be queried for new drops and can be ignored in future Redis rebuilds for that group.