refactor: consolidate API to server_api and remove deprecated stacks

[CaralHsi]

Summary

• Consolidate API entrypoints to the server_api path and remove deprecated start_api / product_api surfaces.
• Remove unused legacy modules (nebular graph implementation and old mem_os product wrappers) to reduce maintenance overhead and duplicate runtime paths.
• Align CLI / Makefile / docker and handler initialization with the server-only path.

Why

The branch had multiple historical API/runtime paths that overlapped with current server_api behavior and increased ambiguity about supported interfaces. This cleanup keeps one primary server path and removes dead/deprecated implementations.

Scope of changes

• Deleted deprecated API modules and tests:
  • src/memos/api/start_api.py
  • src/memos/api/product_api.py
  • src/memos/api/routers/product_router.py
  • tests/api/test_start_api.py
  • tests/api/test_product_router.py
• Deleted deprecated graph/product modules:
  • src/memos/graph_dbs/nebular.py
  • src/memos/mem_os/product.py
  • src/memos/mem_os/product_server.py
  • src/memos/mem_os/client.py
• Updated references and initialization paths:
  • Makefile serve target -> memos.api.server_api:app
  • src/memos/cli.py / tests/test_cli.py OpenAPI export path updates
  • docker/Dockerfile.krolik entrypoint alignment
  • handler dependency/init cleanup for removed mos_server
  • related config/factory updates to remove deleted backends

Validation

• python -m pytest tests/test_cli.py -q -> passed
• compileall on edited modules passed
• Note: some integration tests are environment-dependent and still require optional modules such as redis / psycopg2.

Closes #1346