Migrate notifications to gRPC and fetch proto via buf (no retries)

[rowan-stein]

User request

• Migrate the notifications service to gRPC.
• Do not commit schema files in the repo; fetch proto definitions using buf from agynio/api.
• Do not add any retry logic for now.
• Keep UI exposure via Socket.IO through gateway; notifications service remains internal.

Implementation specification

• Proto source: buf module buf.build/agynio/api (notifications v1 in proto/agynio/api/notifications/v1/notifications.proto; see PR feat(notifications): add gRPC contract (v1) api#4).
• Codegen: add a proto:gen script that runs then using connect-es + es plugins to generate TS stubs into . Do not commit generated files.
• Notifications service (Node): implement gRPC server for and using @connectrpc/connect-node. Validate publish payload against existing zod schemas; generate id/ts server-side; stream live envelopes to subscribers. Keep Socket.IO serving UI as-is during transition.
• Platform-server: replace HTTP NotificationsClient with gRPC client using connect-node; no retries; set per-request reasonable timeout if needed (but no retry/backoff).
• CI: install/run buf; run proto:gen before build/test; ensure generated code resides under .gen/ and is not committed.
• Compose: expose notifications gRPC port; set NOTIFICATIONS_GRPC_ADDR for platform-server to target notifications.

Acceptance criteria

• gRPC server for notifications runs locally and in compose; Publish/Subscribe work end-to-end.
• Platform-server uses gRPC client (no HTTP path) and successfully publishes.
• No retries implemented in client.
• Buf-based proto fetch and codegen run in CI and local dev; no proto files committed.
• Socket.IO UI continues to function via gateway.