feat: add standalone Flow RecaptchaV3TaskProxylessM1 captcha API

[yubanmayi]

Motivation

• Provide a dedicated, OpenAI-style endpoint to request Flow reCAPTCHA v3 tokens through the browser captcha provider, centralizing Flow browser captcha calls.
• Ensure operator-configurable upstream service integration (URL, path, API key, timeout) so deployments can point to an internal/third-party Flow captcha service.

Description

• Added POST /v1/captcha/flow-recaptcha-v3-task-proxyless-m1 handler with request model FlowRecaptchaV3TaskProxylessM1Request that accepts project_id or website_url and page_action (IMAGE_GENERATION or VIDEO_GENERATION).
• The endpoint calls the configured upstream Flow captcha service (via curl_cffi AsyncSession) using Bearer auth, configurable timeout, and unified success/error response shapes; the success payload includes token, duration_ms, task_type and fixed pricing fields (0.015 per task / 15/1000).
• Added runtime config accessors in src/core/config.py for flow_captcha_service_base_url, flow_captcha_service_solve_path, flow_captcha_service_api_key, and flow_captcha_service_timeout_seconds with sensible defaults.
• Updated config/setting_example.toml with the new captcha config keys and extended the README.md with example curl usage and ops notes.

Testing

• Ran python -m compileall src to verify the project compiles; compilation completed successfully.

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Codex Task