fix(security): close every fixable advisory and make the unfixable one unreachable

The dependency scan reported 14 high/critical findings across the backend and
the marketplace frontend. All of them are now closed, and the scan is green for
the first time.

**Bumped, with the suite as the check.** aiohttp 3.14.1 -> 3.14.3, pyasn1 0.6.3
-> 0.6.4, pydantic-settings 2.14.1 -> 2.15.0, and cryptography 48.0.1 -> 50.0.0.
That last one crosses two majors, which is why it was flagged as blast radius
rather than a routine bump; the full backend suite passes unchanged. nanoid and
postcss in the marketplace frontend are patched and the frontend still builds.

**ecdsa has no fix and never will.** CVE-2024-23342 is a Minerva timing attack on
P-256, and the project considers side channels out of scope. It arrives through
python-jose, and only signing, key generation and ECDH are affected —
verification is not. The backend signs tenant tokens with the symmetric
JWT_SECRET, so only HMAC families are coherent there anyway.

That was true by habit, not by construction: `jwt_algorithm` had no validation at
all, so JWT_ALGORITHM=ES256 would have signed through the vulnerable path with
nothing to say so. app/core/security.py now refuses any algorithm outside
HS256/HS384/HS512, on both the encode and decode paths, and
tests/test_jwt_algorithm_guard.py fails if that control is removed. `none` is
refused alongside ES*: an unsigned token is not a lesser problem than a badly
signed one.

The advisory is accepted by exact ID with that control named, using a mechanism
added here rather than by silencing the tool. A new advisory on ecdsa still
fails, and a package whose every finding is accepted stops being listed as
vulnerable so the field keeps meaning something.

Backend 1419 passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

zk/go.sum

@@ -0,0 +1,2 @@
+github.com/gtank/ristretto255 v0.1.2 h1:JEqUCPA1NvLq5DwYtuzigd7ss8fwbYay9fi4/5uMzcc=
+github.com/gtank/ristretto255 v0.1.2/go.mod h1:Ph5OpO6c7xKUGROZfWVLiJf9icMDwUeIvY4OmlYW69o=