Remove seeds/ folder after memory migration

[ryaneggz]

Context

PR #800 (merged) migrated the memory seeder into the Docker build so it can run from deployed environments. Now that the migration is complete, the backend/seeds/ directory is no longer needed and should be cleaned up.

Current State

backend/seeds/ contains:

• memory_seeder.py — migrated via FROM feat/787-migrate-memories-seeder TO development #800
• user_seeder.py — referenced in docs/CLAUDE.md (make seeds.user)

Tasks

• Verify memory_seeder.py is no longer invoked outside of the Docker container
• Verify user_seeder.py is no longer needed or migrate it if still in use
• Remove backend/seeds/ directory
• Update CLAUDE.md references to seeds.user / python -m seeds.user_seeder
• Update Makefile if seeds.user target exists
• Update Dockerfile / .dockerignore if they reference seeds/