Release v0.28.0: security-audit-0026 I3 + I4 (HTML-email guard + async Turnstile)

Two informational findings from the Session-0026 audit, both
framework-internal defense-in-depth. No operator action: no migration,
no schema/config/overlay change, no deployment-facing surface.

- I3: guard the dead text/html branch in email_envelope.build_envelope.
  No send path passes body_html; the unused branch would emit HTML built
  from possibly-unescaped user content (C1 stored-XSS class in the mail
  channel). Passing body_html now raises NotImplementedError; the arg is
  kept for documented future symmetry, enabling HTML mail becomes a
  deliberate escape-then-unguard change.

- I4: make turnstile.verify_token async. The sync httpx.post ran inside
  the async /auth/otc/request handler, blocking the event loop up to the
  10s timeout on a slow CloudFlare call. It now awaits httpx.AsyncClient
  via a narrow _siteverify_post seam (tests patch the seam, not the
  shared AsyncClient). The sole caller (main.py) now awaits it.

Tests: full backend suite 365 passed. Added a coroutine-contract unit
test for verify_token and flipped the email_envelope HTML test to assert
the guard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/turnstile.py

@@ -73,6 +73,19 @@ def _siteverify_url() -> str:
     return os.environ.get("TURNSTILE_SITEVERIFY_URL", "").strip() or SITEVERIFY_URL
 
 
+async def _siteverify_post(url: str, data: dict) -> httpx.Response:
+    """Perform the siteverify POST on an `httpx.AsyncClient`.
+
+    Isolated as a narrow seam (I4, security-audit-0026): the call is
+    awaited so a slow CloudFlare response can't block the event loop,
+    and tests patch *this function* rather than the shared
+    `httpx.AsyncClient` (which other modules — gitea, docs — also
+    construct, so a global patch would break app boot).
+    """
+    async with httpx.AsyncClient(timeout=10.0) as client:
+        return await client.post(url, data=data)
+
+
 @dataclass
 class VerifyOutcome:
     """Result of a Turnstile siteverify call.
@@ -98,7 +111,7 @@ class VerifyOutcome:
     reason: str
 
 
-def verify_token(token: str | None, *, client_ip: str | None = None) -> VerifyOutcome:
+async def verify_token(token: str | None, *, client_ip: str | None = None) -> VerifyOutcome:
     """Validate a Turnstile token against CloudFlare's siteverify endpoint.
 
     Returns a VerifyOutcome describing whether the calling endpoint
@@ -108,9 +121,14 @@ def verify_token(token: str | None, *, client_ip: str | None = None) -> VerifyOu
       * 'misconfigured' → 500 "auth misconfigured"
       * 'missing-token' / 'failed' / 'network' → 400 "verification failed"
 
-    Tests monkeypatch `httpx.post` (or set `TURNSTILE_SITEVERIFY_URL`
-    + a MockTransport client) to avoid touching the real CloudFlare
-    endpoint. No real keys are ever embedded in tests.
+    Async (I4, security-audit-0026): the siteverify call is awaited on an
+    `httpx.AsyncClient` so a slow CloudFlare response can't block the
+    event loop (the prior synchronous `httpx.post` stalled the single
+    worker for up to the 10s timeout). Callers must `await` it.
+
+    Tests monkeypatch `_siteverify_post` (the narrow async seam) to avoid
+    touching the real CloudFlare endpoint and to keep the patch off the
+    shared `httpx.AsyncClient`. No real keys are ever embedded in tests.
     """
     secret = _secret()
     required = _required()
@@ -133,7 +151,7 @@ def verify_token(token: str | None, *, client_ip: str | None = None) -> VerifyOu
         data["remoteip"] = client_ip
 
     try:
-        response = httpx.post(_siteverify_url(), data=data, timeout=10.0)
+        response = await _siteverify_post(_siteverify_url(), data)
         payload = response.json()
     except Exception as exc:  # network, JSON parse, etc.
         log.warning("Turnstile siteverify call failed: %s", exc)