feat(hub): replace reCAPTCHA with a self-hosted proof of work

Sign-up and passphrase reset sent every visitor's address and browser
details to Google, which needed a consent the sign-up form could not ask
for freely. The hub now issues its own challenge (ALTCHA's scheme, signed
with a key derived from the hub's private key, single-use, 30 minutes)
and the page solves it with SubtleCrypto while the form is filled in.

- GET /v1/users/captcha issues a challenge; 404 captcha_disabled when off.
- [captcha] enabled / max_number replace site_key / secret_key, which are
  now ignored with a warning; MESHBAY_CAPTCHA_ENABLED replaces the old
  variables.
- No Google origin left in the CSP of the hub, the desktop shell or the
  Android shell; captcha_site_key leaves /v1/hub/info.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 0 insertions, 10 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/hub.py b/packages/meshbay-hub/src/meshbay_hub/api/hub.py
index 0d2d39c..b47f076 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/hub.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/hub.py
@@ -7,19 +7,10 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from meshbay_hub import __version__, hub_settings
 from meshbay_hub.api import federation
 from meshbay_hub.auth import hub_public_key_pem
-from meshbay_hub.config import HubConfig
 from meshbay_hub.db.engine import get_db, get_engine
 
 router = APIRouter(prefix="/v1/hub", tags=["hub"])
 
-_cfg: HubConfig | None = None
-
-
-def set_config(cfg: HubConfig) -> None:
-    global _cfg
-    _cfg = cfg
-
-
 @router.get("/info")
 async def hub_info(db: AsyncSession = Depends(get_db)):
     """Versions and the instance policy a client needs before signing in."""
@@ -40,7 +31,6 @@ async def hub_info(db: AsyncSession = Depends(get_db)):
         # which answers 503 — this is the same answer somewhere an operator can
         # read it without opening the source.
         "federation": federation.FEDERATION_ENABLED,
-        "captcha_site_key": _cfg.captcha.site_key if _cfg and _cfg.captcha.enabled else "",
         # How long a browser tab stays signed in with nobody at it. The page
         # measures it (static/idle.js); the hub only says how long.
         "browser_idle_hours":