feat(parser): add entrypoint infrastructure for multiple stream types

[cchwala]

What

Splits the parser into pluggable entrypoints to support different data ingestion loops (SFTP push, SFTP pull, API pull) while reusing the same parser infrastructure.

Changes

New entrypoint modules

• parser/entrypoints/sftp_push — Current logic (watch local directory for files)
• parser/entrypoints/sftp_pull — Stub for future remote SFTP polling
• parser/entrypoints/api_pull — Stub for future HTTP/API polling

Core changes

• parser/main.py becomes a thin shim that delegates to sftp_push entrypoint
• generate_config.py emits command: based on new entrypoint field in users.yml
• Validation added for entrypoint values (sftp_push, sftp_pull, api_pull)
• users.yml schema extended with entrypoint documentation

Tests

• All 90 parser tests passing
• Test imports updated to reference parser.entrypoints.sftp_push

Configuration

New optional field in users.yml:

sources:
  - id: main
    parser: demo_csv_data
    entrypoint: sftp_push  # default, can be omitted

Future stream types will use:

• entrypoint: sftp_pull — for remote SFTP polling
• entrypoint: api_pull — for HTTP/API polling

Backward compatibility

Existing services default to sftp_push entrypoint when not specified. No breaking changes to deploy repo configuration.