Feature: Add tier-based provider prioritization

[etwk]

Add support for configuring providers in priority tiers, where lower-tier (higher priority) providers are used first, and higher-tier providers serve as fallbacks only when all lower-tier providers are exhausted.

Use Case

Many users have multiple API access methods with different cost structures:

• Tier 1: Subscription-based accounts (e.g., Claude Max) - cheaper/included usage
• Tier 2: Usage-based API keys (e.g., pay-per-token) - more expensive

Currently, CLIProxyAPI uses round-robin across all providers equally, which means expensive usage-based credentials get consumed even when cheaper subscription-based options are available.

Proposed Configuration

claude-api-key:
  - api-key: "AIzaSy...subscription"
    tier: 1  # Use first (subscription-based, cheaper)
  - api-key: "AIzaSy...payasyougo"
    tier: 2  # Fallback only (usage-based, expensive)

openai-compatibility:
  - name: "openrouter"
    tier: 2  # Fallback provider
    # ...
  - name: "local-llm"
    tier: 1  # Preferred provider
    # ...

Expected behavior:

1. Always prefer tier 1 providers (round-robin within tier 1)
2. Only use tier 2 when ALL tier 1 providers are:
   • Quota exceeded (in cooldown)
   • Temporarily unavailable (auth errors, rate limits)
3. Automatically return to tier 1 when any tier 1 provider recovers
4. Default tier: 1 if not specified (backward compatible)
5. Able to add more tiers (tier 3, tier 4, etc.) following the same pattern

Alternatives considered:

• Manually disabling expensive credentials and re-enabling when needed (inconvenient)
• Running separate proxy instances per tier (complex, doesn't solve automatic fallback)