Improve response, audience-hint, and upload-error rendering

[Shayanide]

Stacked on #21 (base: feat/credits-display) so these land together under the same 0.3.0.

• Trim audience-filter hints to the geography keys the API currently supports for external clients.
• Render the API's response labels in raw responses (falls back to the raw value when absent).
• Parse structured media-upload errors and map them to clear messages instead of dumping the raw body.

Test plan:

• uv run ruff check mcp_server tests
• uv run pytest tests/ -v — 153 passed