Add startup config logging, /config endpoint, and Makefile

[Copilot]

The API/CLI lacked visibility into runtime configuration, making it difficult to debug mode/auth/project issues without inspecting code or environment variables.

Changes

• Startup banner: Logs MUSIC_GEN_MODE, API key status (yes/no), GCP project/region at API startup. No secrets in logs.

• GET /config endpoint: Returns safe config snapshot without credentials:

  {
    "mode": "simulate",
    "region": "us-central1",
    "project": "my-project",
    "presets_available": ["rock_anthem", "jazz_smooth", ...],
    "auth_enabled": true
  }

• Structured validation errors: Pydantic returns field-specific 422 errors with location and constraints for duration/temperature validation.

• Makefile: Targets for venv, install, test, api, cli-help workflows.

• UTF-8 logging: Configured stdout reconfigure for emoji/international character support.

• Updated README: Added Makefile usage, expected CLI outputs, /config endpoint docs.

Technical notes

• Replaced deprecated @app.on_event("startup") with lifespan context manager
• Test fixtures refactored to avoid module reload side effects
• All existing endpoints unchanged (backward compatible)

Original prompt

You are a coding agent working in the repository "github-dev-sandbox" (Music Track Generator).

Goal: make the API/CLI behavior predictable and self-documenting, especially around environment configuration, without changing core features.

Constraints:

• Keep current endpoints stable.
• Do not implement a real Vertex AI music model call yet.
• No silent fallbacks: print clear logs for mode/auth/config at startup.

Tasks:

1. Add a startup log banner in api.py that prints:
   • MUSIC_GEN_MODE value
   • whether MUSIC_GEN_API_KEY is set (yes/no only)
   • GOOGLE_CLOUD_PROJECT and GOOGLE_CLOUD_REGION when in gcp mode
2. Add a /config endpoint (GET) that returns safe config info:
   • mode, region, project (if any), presets available, auth enabled true/false
   • never return secrets
3. Improve error handling for /tracks/generate:
   • validate duration range and return a structured 422 error with field names
4. Add a Makefile with targets:
   • venv, install, test, api, cli-help
5. Update README with exact commands and expected outputs for simulate mode.

Deliver:

• A single PR with tests updated/added where needed (pytest).
• Ensure UTF-8 logs and no secrets in logs.

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