QONTO-WP-0004-T03: verify key-cape IAM Profile tokens

Replaces the interim shared-secret bearer token's role as the identity
boundary with real key-cape JWKS-based verification, closing the gap
docs/mcp-integration.md called out explicitly ("no OIDC issuer exists
in this fleet yet") -- key-cape's /jwks is a standard RS256 endpoint
and needed no key-cape-side work to consume.

KeyCapeTokenVerifier fetches and caches signing keys over httpx
(matching FlexAuthCheckClient's pattern elsewhere in this codebase),
validates iss/aud/exp and the IAM Profile v0.3 required claims, and
derives ActorClaims from the token (tenant, scopes, and a lane
inferred from the roles claim). Wired into auth.actor_claims_from_headers,
the single seam both REST and MCP already used -- a verified bearer
token now takes precedence over self-asserted X-Actor-* headers, and
can be made mandatory via QONTO_KEY_CAPE_REQUIRED once real tokens are
issued to callers. Off by default (no QONTO_KEY_CAPE_JWKS_URL set) so
existing deployments are unaffected until configured.

The QONTO_ASSISTANT_MCP_TOKEN shared secret remains as a documented
local-dev/legacy fallback, not the auth boundary going forward.

Verified: 13 new tests (test_key_cape_auth.py) covering valid/expired/
wrong-audience/wrong-issuer/missing-claim/unknown-key/rotated-key
tokens plus the auth.py precedence and required-vs-optional
integration paths, using a real generated RSA keypair and JWKS served
over httpx.MockTransport. Full suite -> 52 passed; REST and MCP smokes
both still pass against fixtures; compileall clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pyproject.toml

@@ -14,6 +14,7 @@ dependencies = [
     "fastapi>=0.115,<1.0",
     "httpx>=0.27,<1.0",
     "mcp>=1.9,<2.0",
+    "PyJWT[crypto]>=2.9,<3.0",
     "PyYAML>=6.0,<7.0",
     "uvicorn[standard]>=0.30,<1.0",
 ]