fix: resolve issues #73, #75, #79 (indexes, notifications, event filtering)

[solomon35-stack]

Summary

Fixes three open issues assigned to @solomon35-stack.

#73 — Add indexes for common query patterns

• idx_streams_user_id_status on streams(user_id, status) for dashboard queries
• idx_stream_events_stream_id_occurred_at on stream_events(stream_id, created_at DESC) for history queries
• idx_users_email unique index on users(email) for auth lookups
• Rollback: DROP INDEX statements noted in schema comments

#75 — Add notifications table

• Columns: id, user_id (FK → users), type, payload (JSONB), read_at, created_at
• idx_notifications_user_id for general user lookups
• idx_notifications_unread partial index (WHERE read_at IS NULL) for unread-count queries
• Rollback: DROP TABLE notifications noted in schema comments

#79 — Configurable event filtering

• New EventFilter class in xstreamroll-processing/src/pipeline/
• Per-stream block-list of event types loaded via setConfig(streamId, config)
• Filtered events are dropped silently (no dead-letter)
• Hot-reloadable: call setConfig at any time without restarting the worker
• Wired into the worker poll loop before events are routed to sessions

Testing

• TypeScript compiles clean (tsc --noEmit)

Closes #73, #75, #79