Gate Flow CAPTCHA generation after account-safety reports

[raducupreda-art]

Issue #67 reports two Google accounts unable to generate even in the manual Flow UI after FlowKit testing. That timing is concerning, but it does not establish that FlowKit or PR #64 caused the restriction.

Before this change, FlowKit paused for only 30/120 seconds after hijack or unusual-activity signals; the worker could requeue hijack failures without counting a retry, and generic CAPTCHA failures could be retried repeatedly. The extension also auto-injected the hijack bypass and could mint more than once.

This draft makes CAPTCHA-bearing generation opt-in (FLOW_ENABLE_CAPTCHA_GENERATION=1) and serializes it. It writes a durable in-flight marker before requesting a token, then keeps a persistent safety hold after unusual-activity, hijack, CAPTCHA, or uncertain transport failures. The hold survives restart and cannot be cleared by /api/flow/clear-hijack. The extension no longer auto-loads the hijack bypass or a second reCAPTCHA runtime, uses at most one public mint attempt, and rejects tokens returned by stale tabs without a verified mint-path diagnostic. Read-only media and polling RPCs remain available. README, dashboard, extension, and troubleshooting docs warn testers and distinguish extension-only failures from a manual Flow UI block; no Google recovery time is claimed.

Offline validation: 275 Python unit tests (excluding three unrelated Windows/environment-dependent modules), 10 Node extension tests, JS syntax checks, and git diff --check passed. The full Python unit run surfaced existing setup/CLI encoding and Windows symlink failures outside the changed paths. One bounded FlowKit canary was attempted in AdsPower FLOW 1 (one still, one attempt, no references). The patched extension returned EXTENSION_HIJACK_DETECTED before token minting or a generation RPC; the server persisted an extension_hijack_detected hold and stopped further requests. Read-only page inspection showed no old FlowKit bypass object (window.__fk_hijack absent), while the page's public execute wrapper contained the extension_hijack_detected marker. No image was generated. This validates the fail-closed path, not generation success; manual Flow UI access on this account was not established by this test.

This is a temporary safety mitigation, not a root-cause fix. The opted-in mint path needs separate live verification using a disposable account before any release.

[crisng95]

wait me for testing