Release v0.15.2-hjallr.4

This release includes:

- fix/appservice-space-members-join: Fix HTTP 500 error for appservice users joining restricted rooms
- fix/max-depth-cap: Cap event depth at MAX_SAFE_INTEGER to prevent canonical JSON overflow
- fix/receipt-sequence-race: Fix receipt sequence ID race causing badges to not clear
- fix/error-code-compliance: Improve Matrix error code compliance across APIs

- msc3266-room-summary: Room Summary API (Phases 1-4)
- msc3706-faster-joins: Partial state joins for faster room joins
- msc4115-membership-on-events: Membership on events support
- sliding-sync: Simplified Sliding Sync (MSC4186) implementation

Note: This release adds new tables and columns. Rollback to v0.15.2-hjallr.3 is safe - the old version will ignore the new schema elements.