[codex] Disable client fast mode requests

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What changed

• Added CODEX_DISABLE_FAST_MODE / DISABLE_FAST_MODE runtime env support.
• When enabled, client fast-tier requests are ignored instead of forwarded upstream:
  • OpenAI service_tier: "fast" and service_tier: "priority" are removed before upstream dispatch.
  • Anthropic /v1/messages speed: "fast" no longer maps to Codex service_tier: "priority".
• Updated service-tier extraction so usage context/logging does not continue to mark those requests as fast when the server disables fast mode.
• Added startup logging for disable_fast_mode.
• Documented the new env var in env examples and configuration docs.

Why

The server needs an operator-side override so clients cannot force fast/priority behavior simply by sending request parameters.

Validation

• Ran go test ./proxy locally.
• Built and deployed a patched image on the production host.
• Verified production logs show disable_fast_mode=true and https://sub.laoliu.co/admin/ returns HTTP 200.

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