Key management

[sm86]

Allow users to have multiple keys and do key management well.

If on claude code and I am using it, I want to run out of subscription credits before i use my other credits.

[Shriiii01]

Implemented in PR #74.

• Multiple keys per provider with priority ordering; OAuth/subscription tokens are used first, then keys by priority.
• Key management API: GET /keys, POST /keys, DELETE /keys/:id, PUT /keys/:id/priority.
• Env vars are auto-seeded into the store, so existing setups keep working; you can add more keys via the API and set lower priority for pay-per-use keys so subscription credits are used first.
• Passthroughs resolve keys through the key manager with automatic fallback when a key is exhausted.

Ready for review.