New Added

- Added clan point economy: kills grant configurable clan points and are persisted across all storage backends.
- Introduced member slot upgrades: new ``/clan slots [buy]`` (perm: ``vanguardclans.user.slots``) with base slots, per-upgrade slot gains, and optional static limits.
- Enforced clan capacity: joining/inviting now respects slot limits and blocks when full.
- Admin control: ``/clanadmin points <clan> <amount>`` to grant clan points.


Bug fixes
- PlaceholderAPI usage is now guarded to avoid crashes if PAPI is missing or fails.