Feat/notifications: In-app notification system with placed inboxes

[St4NNi]

Adds the backend of the in-app notification system as one shared primitive: mutating operations emit ephemeral post-commit events, routing mints recipient-addressed records, and a durable per-origin FIFO outbox delivers them to a deterministic per-user inbox holder over a dedicated ALPN. Any node serves the REST API by proxying one hop to the holder, so the portal stays topology-blind.

Stacked on #306 (feat/public-access); diff shows only the notification commits.

Changes

• Adds notification core structs, kinds, and keyspaces: ephemeral events, recipient-addressed records, and per-user inbox rows.
• Adds a post-commit emission helper so events are emitted only after the host transaction commits and the notification path can never fail the host operation.
• Resolves a deterministic per-user inbox holder over sync rendezvous (R=1) so every node agrees where a user's inbox lives.
• Adds a durable per-origin FIFO outbox with local delivery for records whose holder is the origin node.
• Adds a notification delivery RPC over a dedicated ALPN for records whose holder is remote.
• Drains the entire outbox each run to avoid head-of-line starvation behind an unreachable holder.
• Adds notification read and mark-read operations on the holder.
• Adds GET /api/v1/notifications with cursor pagination, GET /api/v1/notifications/unread, and POST /api/v1/notifications/read, each served from any node via holder proxy.
• Adds a TTL prune task: direct notifications expire after 90 days, transient ones after 30 days, plus a per-user inbox cap.
• Emits notifications for group membership changes and node onboarding.
• Adds multi-node delivery integration tests, plus coverage for the remote dispatch arms from a non-holder node.
• Adds the design document docs/design/notifications.md.
• Upgrades workspace dependencies.

GitHub Issues

• Backend for [feat] Notification system #251. The portal bell and badge UI lands in the website repo.
• Delivery upgrades (email, live stream) are [feat] Notification delivery: email and live stream #288 and resource watches are [feat] User resource watches #304; both land separately on top of this.