feat: upgrade to gemini-webapi 2.0.0 with Gemini 3 model support & Expose Gemini Gems via REST endpoints

[hiyukoim]

Change 1: Gemini 3 Model Support (gemini-webapi 2.0.0)

Summary

• Upgrade gemini-webapi from 1.8.3 to >= 2.0.0 to add Gemini 3 model support
• Update GeminiModels enum with all available Gemini 3 models: gemini-3-pro, gemini-3-flash, gemini-3-flash-thinking, plus Plus and Advanced tier variants
• Relax pinned dependency versions (pydantic, curl-cffi, cffi, typing-extensions, pydantic-core) to resolve conflicts with gemini-webapi 2.0.0
• Update default model to gemini-3-flash
• Update config.conf.example with current available model list

Breaking Change

gemini-webapi 2.0.0 drops older model identifiers (gemini-1.5-*, gemini-2.0-*, gemini-2.5-*).
Only Gemini 3 series models are available with this upgrade. This is an upstream library change. The old model names no longer resolve on the Gemini web API.

Testing

Tested on a headless Ubuntu 24.04 server (Docker) with cookie-based authentication:

• ✅ /v1/chat/completions with gemini-3-flash and gemini-3-pro
• ✅ /gemini endpoint (single-turn)
• ✅ /translate endpoint
• ✅ Cookie rotation (read-write config)
• ✅ Gemini client initialisation with gemini-webapi 2.0.0

Known Issue

/gemini-chat session persistence does not work correctly. The chat ID resets between messages, so the model loses context. This appears to be an upstream issue in gemini-webapi 2.0.0 (ChatSession.cid changes on each send_message call) and is not introduced by this PR. An upstream issue will be filed separately.

Change 2: Expose Gems Support (via gemini-webapi)

Summery

• Add optional gem parameter to all endpoints (/gemini, /gemini-chat,
  /v1/chat/completions)
• New /v1/gems endpoint to list available gems (predefined + custom)
• Pass a gem ID or name to use Gemini Gems as system prompts

POST /v1/chat/completions
{
  "model": "gemini-3-flash",
  "messages": [{"role": "user", "content": "hello"}],
  "gem": "brainstormer"
}

• ✅ /v1/gems listing (predefined + custom)
• ✅ Chat with predefined gem
• ✅ Chat with custom gem
• ✅ Backward compatible (gem parameter is optional)